diff --git a/Lib/ctypes/test/test_bitfields.py b/Lib/ctypes/test/test_bitfields.py
index e9c46c2..090fb4d 100644
--- a/Lib/ctypes/test/test_bitfields.py
+++ b/Lib/ctypes/test/test_bitfields.py
@@ -240,5 +240,25 @@
             _anonymous_ = ["_"]
             _fields_ = [("_", X)]
 
+    @unittest.skipUnless(hasattr(ctypes, "c_uint32"), "c_int32 is required")
+    def test_uint32(self):
+        class X(Structure):
+            _fields_ = [("a", c_uint32, 32)]
+        x = X()
+        x.a = 10
+        self.assertEquals(x.a, 10)
+        x.a = 0xFDCBA987
+        self.assertEquals(x.a, 0xFDCBA987)
+
+    @unittest.skipUnless(hasattr(ctypes, "c_uint64"), "c_int64 is required")
+    def test_uint64(self):
+        class X(Structure):
+            _fields_ = [("a", c_uint64, 64)]
+        x = X()
+        x.a = 10
+        self.assertEquals(x.a, 10)
+        x.a = 0xFEDCBA9876543211
+        self.assertEquals(x.a, 0xFEDCBA9876543211)
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS b/Misc/NEWS
index 6a2abb5..c5fc1bd 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -87,6 +87,9 @@
 Library
 -------
 
+- Issue #6493: An issue in ctypes on Windows that caused structure bitfields
+  of type ctypes.c_uint32 and width 32 to incorrectly be set has been fixed.
+
 - Issue #14635: telnetlib will use poll() rather than select() when possible
   to avoid failing due to the select() file descriptor limit.
 
diff --git a/Modules/_ctypes/cfield.c b/Modules/_ctypes/cfield.c
index 1aab202..5e0fc75 100644
--- a/Modules/_ctypes/cfield.c
+++ b/Modules/_ctypes/cfield.c
@@ -431,12 +431,8 @@
 #define LOW_BIT(x)  ((x) & 0xFFFF)
 #define NUM_BITS(x) ((x) >> 16)
 
-/* This seems nore a compiler issue than a Windows/non-Windows one */
-#ifdef MS_WIN32
-#  define BIT_MASK(size) ((1 << NUM_BITS(size))-1)
-#else
-#  define BIT_MASK(size) ((1LL << NUM_BITS(size))-1)
-#endif
+/* Doesn't work if NUM_BITS(size) == 0, but it never happens in SET() call. */
+#define BIT_MASK(type, size) (((((type)1 << (NUM_BITS(size) - 1)) - 1) << 1) + 1)
 
 /* This macro CHANGES the first parameter IN PLACE. For proper sign handling,
    we must first shift left, then right.
@@ -448,10 +444,10 @@
     }
 
 /* This macro RETURNS the first parameter with the bit field CHANGED. */
-#define SET(x, v, size)                                                 \
+#define SET(type, x, v, size)                                                 \
     (NUM_BITS(size) ?                                                   \
-     ( ( x & ~(BIT_MASK(size) << LOW_BIT(size)) ) | ( (v & BIT_MASK(size)) << LOW_BIT(size) ) ) \
-     : v)
+     ( ( (type)x & ~(BIT_MASK(type, size) << LOW_BIT(size)) ) | ( ((type)v & BIT_MASK(type, size)) << LOW_BIT(size) ) ) \
+     : (type)v)
 
 /* byte swapping macros */
 #define SWAP_2(v)                               \
@@ -523,7 +519,7 @@
     long val;
     if (get_long(value, &val) < 0)
         return NULL;
-    *(signed char *)ptr = (signed char)SET(*(signed char *)ptr, (signed char)val, size);
+    *(signed char *)ptr = SET(signed char, *(signed char *)ptr, val, size);
     _RET(value);
 }
 
@@ -542,8 +538,7 @@
     unsigned long val;
     if (get_ulong(value, &val) < 0)
         return NULL;
-    *(unsigned char *)ptr = (unsigned char)SET(*(unsigned char*)ptr,
-                                               (unsigned short)val, size);
+    *(unsigned char *)ptr = SET(unsigned char, *(unsigned char*)ptr, val, size);
     _RET(value);
 }
 
@@ -564,7 +559,7 @@
     if (get_long(value, &val) < 0)
         return NULL;
     memcpy(&x, ptr, sizeof(x));
-    x = SET(x, (short)val, size);
+    x = SET(short, x, val, size);
     memcpy(ptr, &x, sizeof(x));
     _RET(value);
 }
@@ -579,7 +574,7 @@
         return NULL;
     memcpy(&field, ptr, sizeof(field));
     field = SWAP_2(field);
-    field = SET(field, (short)val, size);
+    field = SET(short, field, val, size);
     field = SWAP_2(field);
     memcpy(ptr, &field, sizeof(field));
     _RET(value);
@@ -612,7 +607,7 @@
     if (get_ulong(value, &val) < 0)
         return NULL;
     memcpy(&x, ptr, sizeof(x));
-    x = SET(x, (unsigned short)val, size);
+    x = SET(unsigned short, x, val, size);
     memcpy(ptr, &x, sizeof(x));
     _RET(value);
 }
@@ -626,7 +621,7 @@
         return NULL;
     memcpy(&field, ptr, sizeof(field));
     field = SWAP_2(field);
-    field = SET(field, (unsigned short)val, size);
+    field = SET(unsigned short, field, val, size);
     field = SWAP_2(field);
     memcpy(ptr, &field, sizeof(field));
     _RET(value);
@@ -660,7 +655,7 @@
     if (get_long(value, &val) < 0)
         return NULL;
     memcpy(&x, ptr, sizeof(x));
-    x = SET(x, (int)val, size);
+    x = SET(int, x, val, size);
     memcpy(ptr, &x, sizeof(x));
     _RET(value);
 }
@@ -674,7 +669,7 @@
         return NULL;
     memcpy(&field, ptr, sizeof(field));
     field = SWAP_INT(field);
-    field = SET(field, (int)val, size);
+    field = SET(int, field, val, size);
     field = SWAP_INT(field);
     memcpy(ptr, &field, sizeof(field));
     _RET(value);
@@ -761,7 +756,7 @@
     if (get_ulong(value, &val) < 0)
         return  NULL;
     memcpy(&x, ptr, sizeof(x));
-    x = SET(x, (unsigned int)val, size);
+    x = SET(unsigned int, x, val, size);
     memcpy(ptr, &x, sizeof(x));
     _RET(value);
 }
@@ -774,7 +769,7 @@
     if (get_ulong(value, &val) < 0)
         return  NULL;
     memcpy(&field, ptr, sizeof(field));
-    field = (unsigned int)SET(field, (unsigned int)val, size);
+    field = SET(unsigned int, field, (unsigned int)val, size);
     field = SWAP_INT(field);
     memcpy(ptr, &field, sizeof(field));
     _RET(value);
@@ -808,7 +803,7 @@
     if (get_long(value, &val) < 0)
         return NULL;
     memcpy(&x, ptr, sizeof(x));
-    x = SET(x, val, size);
+    x = SET(long, x, val, size);
     memcpy(ptr, &x, sizeof(x));
     _RET(value);
 }
@@ -822,7 +817,7 @@
         return NULL;
     memcpy(&field, ptr, sizeof(field));
     field = SWAP_LONG(field);
-    field = (long)SET(field, val, size);
+    field = SET(long, field, val, size);
     field = SWAP_LONG(field);
     memcpy(ptr, &field, sizeof(field));
     _RET(value);
@@ -856,7 +851,7 @@
     if (get_ulong(value, &val) < 0)
         return  NULL;
     memcpy(&x, ptr, sizeof(x));
-    x = SET(x, val, size);
+    x = SET(unsigned long, x, val, size);
     memcpy(ptr, &x, sizeof(x));
     _RET(value);
 }
@@ -870,7 +865,7 @@
         return  NULL;
     memcpy(&field, ptr, sizeof(field));
     field = SWAP_LONG(field);
-    field = (unsigned long)SET(field, val, size);
+    field = SET(unsigned long, field, val, size);
     field = SWAP_LONG(field);
     memcpy(ptr, &field, sizeof(field));
     _RET(value);
@@ -905,7 +900,7 @@
     if (get_longlong(value, &val) < 0)
         return NULL;
     memcpy(&x, ptr, sizeof(x));
-    x = SET(x, val, size);
+    x = SET(PY_LONG_LONG, x, val, size);
     memcpy(ptr, &x, sizeof(x));
     _RET(value);
 }
@@ -919,7 +914,7 @@
         return NULL;
     memcpy(&field, ptr, sizeof(field));
     field = SWAP_8(field);
-    field = (PY_LONG_LONG)SET(field, val, size);
+    field = SET(PY_LONG_LONG, field, val, size);
     field = SWAP_8(field);
     memcpy(ptr, &field, sizeof(field));
     _RET(value);
@@ -952,7 +947,7 @@
     if (get_ulonglong(value, &val) < 0)
         return NULL;
     memcpy(&x, ptr, sizeof(x));
-    x = SET(x, val, size);
+    x = SET(PY_LONG_LONG, x, val, size);
     memcpy(ptr, &x, sizeof(x));
     _RET(value);
 }
@@ -966,7 +961,7 @@
         return NULL;
     memcpy(&field, ptr, sizeof(field));
     field = SWAP_8(field);
-    field = (unsigned PY_LONG_LONG)SET(field, val, size);
+    field = SET(unsigned PY_LONG_LONG, field, val, size);
     field = SWAP_8(field);
     memcpy(ptr, &field, sizeof(field));
     _RET(value);