diff --git a/dlls/kernel/comm.c b/dlls/kernel/comm.c
index 2f1a3fcfd98..dd8770880a5 100644
--- a/dlls/kernel/comm.c
+++ b/dlls/kernel/comm.c
@@ -1975,8 +1975,8 @@ static BOOL COMM_WaitCommEvent(
     commio->fd = fd;  /* FIXME */
 
     lpOverlapped->InternalHigh = 0;
-    lpOverlapped->Offset = 0;
-    lpOverlapped->OffsetHigh = 0;
+    lpOverlapped->u.s.Offset = 0;
+    lpOverlapped->u.s.OffsetHigh = 0;
 
     SERVER_START_REQ( register_async )
     {
diff --git a/dlls/kernel/file.c b/dlls/kernel/file.c
index b1817ac8cf3..8ebdd22adbf 100644
--- a/dlls/kernel/file.c
+++ b/dlls/kernel/file.c
@@ -323,8 +323,8 @@ BOOL WINAPI ReadFileEx(HANDLE hFile, LPVOID buffer, DWORD bytesToRead,
         return FALSE;
     }
 
-    offset.u.LowPart = overlapped->Offset;
-    offset.u.HighPart = overlapped->OffsetHigh;
+    offset.u.LowPart = overlapped->u.s.Offset;
+    offset.u.HighPart = overlapped->u.s.OffsetHigh;
     io_status = (PIO_STATUS_BLOCK)overlapped;
     io_status->u.Status = STATUS_PENDING;
 
@@ -364,8 +364,8 @@ BOOL WINAPI ReadFile( HANDLE hFile, LPVOID buffer, DWORD bytesToRead,
 
     if (overlapped != NULL)
     {
-        offset.u.LowPart = overlapped->Offset;
-        offset.u.HighPart = overlapped->OffsetHigh;
+        offset.u.LowPart = overlapped->u.s.Offset;
+        offset.u.HighPart = overlapped->u.s.OffsetHigh;
         poffset = &offset;
         hEvent = overlapped->hEvent;
         io_status = (PIO_STATUS_BLOCK)overlapped;
@@ -405,8 +405,8 @@ BOOL WINAPI WriteFileEx(HANDLE hFile, LPCVOID buffer, DWORD bytesToWrite,
         SetLastError(ERROR_INVALID_PARAMETER);
         return FALSE;
     }
-    offset.u.LowPart = overlapped->Offset;
-    offset.u.HighPart = overlapped->OffsetHigh;
+    offset.u.LowPart = overlapped->u.s.Offset;
+    offset.u.HighPart = overlapped->u.s.OffsetHigh;
 
     io_status = (PIO_STATUS_BLOCK)overlapped;
     io_status->u.Status = STATUS_PENDING;
@@ -439,8 +439,8 @@ BOOL WINAPI WriteFile( HANDLE hFile, LPCVOID buffer, DWORD bytesToWrite,
 
     if (overlapped)
     {
-        offset.u.LowPart = overlapped->Offset;
-        offset.u.HighPart = overlapped->OffsetHigh;
+        offset.u.LowPart = overlapped->u.s.Offset;
+        offset.u.HighPart = overlapped->u.s.OffsetHigh;
         poffset = &offset;
         hEvent = overlapped->hEvent;
         piosb = (PIO_STATUS_BLOCK)overlapped;
@@ -999,13 +999,13 @@ BOOL WINAPI LockFileEx( HANDLE hFile, DWORD flags, DWORD reserved,
     }
 
     TRACE( "%p %lx%08lx %lx%08lx flags %lx\n",
-           hFile, overlapped->OffsetHigh, overlapped->Offset, 
+           hFile, overlapped->u.s.OffsetHigh, overlapped->u.s.Offset, 
            count_high, count_low, flags );
 
     count.u.LowPart = count_low;
     count.u.HighPart = count_high;
-    offset.u.LowPart = overlapped->Offset;
-    offset.u.HighPart = overlapped->OffsetHigh;
+    offset.u.LowPart = overlapped->u.s.Offset;
+    offset.u.HighPart = overlapped->u.s.OffsetHigh;
 
     status = NtLockFile( hFile, overlapped->hEvent, NULL, NULL, 
                          NULL, &offset, &count, NULL, 
@@ -1050,7 +1050,7 @@ BOOL WINAPI UnlockFileEx( HANDLE hFile, DWORD reserved, DWORD count_low, DWORD c
     }
     if (overlapped->hEvent) FIXME("Unimplemented overlapped operation\n");
 
-    return UnlockFile( hFile, overlapped->Offset, overlapped->OffsetHigh, count_low, count_high );
+    return UnlockFile( hFile, overlapped->u.s.Offset, overlapped->u.s.OffsetHigh, count_low, count_high );
 }
 
 
diff --git a/dlls/kernel/tests/file.c b/dlls/kernel/tests/file.c
index e54f32a28d4..f7ec05c8e97 100644
--- a/dlls/kernel/tests/file.c
+++ b/dlls/kernel/tests/file.c
@@ -28,6 +28,17 @@
 #include "winbase.h"
 #include "winerror.h"
 
+#ifdef NONAMELESSUNION
+# define U(x)  (x).u
+#else
+# define U(x)  (x)
+#endif
+#ifdef NONAMELESSSTRUCT
+# define S(x)  (x).s
+#else
+# define S(x)  (x)
+#endif
+
 static int dll_capable(const char *dll, const char *function)
 {
     HMODULE module = GetModuleHandleA(dll);
@@ -912,8 +923,8 @@ static void test_offset_in_overlapped_structure(void)
     ok(done == sizeof(buf), "expected number of bytes written %lu\n", done);
 
     memset(&ov, 0, sizeof(ov));
-    ov.Offset = PATTERN_OFFSET;
-    ov.OffsetHigh = 0;
+    S(U(ov)).Offset = PATTERN_OFFSET;
+    S(U(ov)).OffsetHigh = 0;
     rc=WriteFile(hFile, pattern, sizeof(pattern), &done, &ov);
     /* Win 9x does not support the overlapped I/O on files */
     if (rc || GetLastError()!=ERROR_INVALID_PARAMETER) {
@@ -923,8 +934,8 @@ static void test_offset_in_overlapped_structure(void)
         ok(SetFilePointer(hFile, 0, NULL, FILE_CURRENT) == (PATTERN_OFFSET + sizeof(pattern)),
            "expected file offset %d\n", PATTERN_OFFSET + sizeof(pattern));
 
-        ov.Offset = sizeof(buf) * 2;
-        ov.OffsetHigh = 0;
+        S(U(ov)).Offset = sizeof(buf) * 2;
+        S(U(ov)).OffsetHigh = 0;
         ret = WriteFile(hFile, pattern, sizeof(pattern), &done, &ov);
         ok( ret, "WriteFile error %ld\n", GetLastError());
         ok(done == sizeof(pattern), "expected number of bytes written %lu\n", done);
@@ -942,8 +953,8 @@ static void test_offset_in_overlapped_structure(void)
 
     memset(buf, 0, sizeof(buf));
     memset(&ov, 0, sizeof(ov));
-    ov.Offset = PATTERN_OFFSET;
-    ov.OffsetHigh = 0;
+    S(U(ov)).Offset = PATTERN_OFFSET;
+    S(U(ov)).OffsetHigh = 0;
     rc=ReadFile(hFile, buf, sizeof(pattern), &done, &ov);
     /* Win 9x does not support the overlapped I/O on files */
     if (rc || GetLastError()!=ERROR_INVALID_PARAMETER) {
@@ -1001,8 +1012,8 @@ static void test_LockFile(void)
     ok( !UnlockFile( handle, 10, 0, 20, 0 ), "UnlockFile 10,20 again succeeded\n" );
     ok( UnlockFile( handle, 5, 0, 5, 0 ), "UnlockFile 5,5 failed\n" );
 
-    overlapped.Offset = 100;
-    overlapped.OffsetHigh = 0;
+    S(U(overlapped)).Offset = 100;
+    S(U(overlapped)).OffsetHigh = 0;
     overlapped.hEvent = 0;
 
     lockfileex_capable = dll_capable("kernel32", "LockFileEx");
@@ -1018,7 +1029,7 @@ static void test_LockFile(void)
     }
 
     /* overlapping shared locks are OK */
-    overlapped.Offset = 150;
+    S(U(overlapped)).Offset = 150;
     limited_UnLockFile || ok( LockFileEx( handle, 0, 0, 100, 0, &overlapped ), "LockFileEx 150,100 failed\n" );
 
     /* but exclusive is not */
@@ -1031,7 +1042,7 @@ static void test_LockFile(void)
         {
             if (!UnlockFileEx( handle, 0, 100, 0, &overlapped ))
             { /* UnLockFile is capable. */
-                overlapped.Offset = 100;
+                S(U(overlapped)).Offset = 100;
                 ok( !UnlockFileEx( handle, 0, 100, 0, &overlapped ),
                     "UnlockFileEx 150,100 again succeeded\n" );
 	    }
@@ -1299,8 +1310,8 @@ static void test_async_file_errors(void)
     HANDLE hFile;
     LPVOID lpBuffer = HeapAlloc(GetProcessHeap(), 0, 4096);
     OVERLAPPED ovl;
-    ovl.Offset = 0;
-    ovl.OffsetHigh = 0;
+    S(U(ovl)).Offset = 0;
+    S(U(ovl)).OffsetHigh = 0;
     ovl.hEvent = hSem;
     completion_count = 0;
     szFile[0] = '\0';
@@ -1317,7 +1328,7 @@ static void test_async_file_errors(void)
         /*printf("Offset = %ld, result = %s\n", ovl.Offset, res ? "TRUE" : "FALSE");*/
         if (!res)
             break;
-        ovl.Offset += 4096;
+        S(U(ovl)).Offset += 4096;
         /* i/o completion routine only called if ReadFileEx returned success.
          * we only care about violations of this rule so undo what should have
          * been done */
diff --git a/dlls/kernel/tests/generated.c b/dlls/kernel/tests/generated.c
index 1d117f6d5bf..55845b6ba89 100644
--- a/dlls/kernel/tests/generated.c
+++ b/dlls/kernel/tests/generated.c
@@ -835,10 +835,10 @@ static void test_pack_OVERLAPPED(void)
 {
     /* OVERLAPPED (pack 4) */
     TEST_TYPE(OVERLAPPED, 20, 4);
-    TEST_FIELD(OVERLAPPED, DWORD, Internal, 0, 4, 4);
-    TEST_FIELD(OVERLAPPED, DWORD, InternalHigh, 4, 4, 4);
-    TEST_FIELD(OVERLAPPED, DWORD, Offset, 8, 4, 4);
-    TEST_FIELD(OVERLAPPED, DWORD, OffsetHigh, 12, 4, 4);
+    TEST_FIELD(OVERLAPPED, ULONG_PTR, Internal, 0, 4, 4);
+    TEST_FIELD(OVERLAPPED, ULONG_PTR, InternalHigh, 4, 4, 4);
+    /*TEST_FIELD(OVERLAPPED, DWORD, Offset, 8, 4, 4);*/
+    /*TEST_FIELD(OVERLAPPED, DWORD, OffsetHigh, 12, 4, 4);*/
     TEST_FIELD(OVERLAPPED, HANDLE, hEvent, 16, 4, 4);
 }
 
diff --git a/dlls/ntdll/loader.c b/dlls/ntdll/loader.c
index de84592ffa1..07f0a972c7c 100644
--- a/dlls/ntdll/loader.c
+++ b/dlls/ntdll/loader.c
@@ -25,6 +25,9 @@
 #include <assert.h>
 #include <stdarg.h>
 
+#define NONAMELESSUNION
+#define NONAMELESSSTRUCT
+
 #include "windef.h"
 #include "winbase.h"
 #include "winnt.h"
diff --git a/dlls/quartz/filesource.c b/dlls/quartz/filesource.c
index 820b57ae8bd..8133d98a8c5 100644
--- a/dlls/quartz/filesource.c
+++ b/dlls/quartz/filesource.c
@@ -18,6 +18,9 @@
  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  */
 
+#define NONAMELESSUNION
+#define NONAMELESSSTRUCT
+
 #include "quartz_private.h"
 
 #include "wine/debug.h"
@@ -966,8 +969,8 @@ static HRESULT WINAPI FileAsyncReader_Request(IAsyncReader * iface, IMediaSample
     {
         DWORD dwLength = (DWORD) BYTES_FROM_MEDIATIME(Stop - Start);
 
-        pDataRq->ovl.Offset = (DWORD) BYTES_FROM_MEDIATIME(Start);
-        pDataRq->ovl.OffsetHigh = (DWORD)(BYTES_FROM_MEDIATIME(Start) >> (sizeof(DWORD) * 8));
+        pDataRq->ovl.u.s.Offset = (DWORD) BYTES_FROM_MEDIATIME(Start);
+        pDataRq->ovl.u.s.OffsetHigh = (DWORD)(BYTES_FROM_MEDIATIME(Start) >> (sizeof(DWORD) * 8));
         pDataRq->ovl.hEvent = This->hEvent;
         pDataRq->dwUserData = dwUser;
         pDataRq->pNext = NULL;
@@ -1120,8 +1123,8 @@ static HRESULT WINAPI FileAsyncReader_SyncRead(IAsyncReader * iface, LONGLONG ll
 
     ovl.hEvent = CreateEventW(NULL, 0, 0, NULL);
     /* NOTE: llPosition is the actual byte position to start reading from */
-    ovl.Offset = (DWORD) llPosition;
-    ovl.OffsetHigh = (DWORD) (llPosition >> (sizeof(DWORD) * 8));
+    ovl.u.s.Offset = (DWORD) llPosition;
+    ovl.u.s.OffsetHigh = (DWORD) (llPosition >> (sizeof(DWORD) * 8));
 
     if (!ReadFile(This->hFile, pBuffer, lLength, NULL, &ovl))
         hr = HRESULT_FROM_WIN32(GetLastError());
diff --git a/dlls/wininet/urlcache.c b/dlls/wininet/urlcache.c
index 337dbe5d4cd..d5c8819ceb7 100644
--- a/dlls/wininet/urlcache.c
+++ b/dlls/wininet/urlcache.c
@@ -32,6 +32,9 @@
 #include <string.h>
 #include <time.h>
 
+#define NONAMELESSUNION
+#define NONAMELESSSTRUCT
+
 #include "windef.h"
 #include "winbase.h"
 #include "winuser.h"
diff --git a/dlls/winsock/socket.c b/dlls/winsock/socket.c
index 2edb58fe77c..a44cbeca07c 100644
--- a/dlls/winsock/socket.c
+++ b/dlls/winsock/socket.c
@@ -2993,7 +2993,7 @@ BOOL WINAPI WSAGetOverlappedResult( SOCKET s, LPWSAOVERLAPPED lpOverlapped,
         *lpcbTransfer = lpOverlapped->InternalHigh;
 
     if ( lpdwFlags )
-        *lpdwFlags = lpOverlapped->Offset;
+        *lpdwFlags = lpOverlapped->u.s.Offset;
 
     switch ( lpOverlapped->Internal )
     {
diff --git a/include/winbase.h b/include/winbase.h
index 3077f66f328..e25409a25da 100644
--- a/include/winbase.h
+++ b/include/winbase.h
@@ -485,10 +485,25 @@ typedef struct _SYSTEMTIME{
 /* The 'overlapped' data structure used by async I/O functions.
  */
 typedef struct _OVERLAPPED {
-        DWORD Internal;
-        DWORD InternalHigh;
-        DWORD Offset;
-        DWORD OffsetHigh;
+#ifdef WORDS_BIGENDIAN
+        ULONG_PTR InternalHigh;
+        ULONG_PTR Internal;
+#else
+        ULONG_PTR Internal;
+        ULONG_PTR InternalHigh;
+#endif
+        union {
+            struct {
+#ifdef WORDS_BIGENDIAN
+                DWORD OffsetHigh;
+                DWORD Offset;
+#else
+                DWORD Offset;
+                DWORD OffsetHigh;
+#endif
+            } DUMMYSTRUCTNAME;
+            PVOID Pointer;
+        } DUMMYUNIONNAME;
         HANDLE hEvent;
 } OVERLAPPED, *LPOVERLAPPED;
 
diff --git a/include/wininet.h b/include/wininet.h
index 8127d4032b6..31b156299a2 100644
--- a/include/wininet.h
+++ b/include/wininet.h
@@ -1366,7 +1366,7 @@ typedef struct _INTERNET_CACHE_ENTRY_INFOA {
     union {
       DWORD dwReserved;
       DWORD dwExemptDelta;
-    } u;
+    } DUMMYUNIONNAME;
 } INTERNET_CACHE_ENTRY_INFOA,* LPINTERNET_CACHE_ENTRY_INFOA;
 
 typedef struct _INTERNET_CACHE_ENTRY_INFOW {
@@ -1389,7 +1389,7 @@ typedef struct _INTERNET_CACHE_ENTRY_INFOW {
     union {
       DWORD dwReserved;
       DWORD dwExemptDelta;
-    } u;
+    } DUMMYUNIONNAME;
 } INTERNET_CACHE_ENTRY_INFOW,* LPINTERNET_CACHE_ENTRY_INFOW;
 
 DECL_WINELIB_TYPE_AW(INTERNET_CACHE_ENTRY_INFO)
diff --git a/include/winnt.h b/include/winnt.h
index 1121fea3fe1..245b1999d53 100644
--- a/include/winnt.h
+++ b/include/winnt.h
@@ -2189,7 +2189,7 @@ typedef struct _IMAGE_IMPORT_DESCRIPTOR {
 	union {
 		DWORD	Characteristics; /* 0 for terminating null import descriptor  */
 		PIMAGE_THUNK_DATA OriginalFirstThunk;	/* RVA to original unbound IAT */
-	} u;
+	} DUMMYUNIONNAME;
 	DWORD	TimeDateStamp;	/* 0 if not bound,
 				 * -1 if bound, and real date\time stamp
 				 *    in IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT