diff --git a/doc/lispref/files.texi b/doc/lispref/files.texi
index ab0dcae2d9c..70c7177e064 100644
--- a/doc/lispref/files.texi
+++ b/doc/lispref/files.texi
@@ -1144,9 +1144,17 @@ return value is unspecified.
 Sometimes file names or their parts need to be compared as strings, in
 which case it's important to know whether the underlying filesystem is
 case-insensitive.  This function returns @code{t} if file
-@var{filename} is on a case-insensitive filesystem.  On platforms where
-this information is not available, this function guesses based on
-common practice.
+@var{filename} is on a case-insensitive filesystem.  It always returns
+@code{t} on MS-DOS and MS-Windows.  On Cygwin and Mac OS X,
+filesystems may or may not be case-insensitive, and the function tries
+to determine case-sensitivity by a runtime test.  If the test is
+inconclusive, the function returns @code{t} on Cygwin and @code{nil}
+on Mac OS X.
+
+Currently this function always returns @code{nil} on platforms other
+than MS-DOS, MS-Windows, Cygwin, and Mac OS X.  It does not detect
+case-insensitivity of mounted filesystems, such as Samba shares or
+NFS-mounted Windows volumes.
 @end defun
 
 @defun file-in-directory-p file dir
diff --git a/src/fileio.c b/src/fileio.c
index eec3591ff6e..f3f8f421618 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -2236,10 +2236,13 @@ internal_delete_file (Lisp_Object filename)
   return NILP (tem);
 }
 
-/* Return true if FILENAME is on a case-insensitive file system.
-   Use a runtime test if available.  Otherwise, assume the file system
-   is case-insensitive on Microsoft-based platforms and case-sensitive
-   elsewhere.
+/* Filesystems are case-sensitive on all supported systems except
+   MS-Windows, MS-DOS, Cygwin, and Mac OS X.  They are always
+   case-insensitive on the first two, but they may or may not be
+   case-insensitive on Cygwin and OS X.  The following function
+   attempts to provide a runtime test on those two systems.  If the
+   test is not conclusive, we assume case-insensitivity on Cygwin and
+   case-sensitivity on Mac OS X.
 
    FIXME: Mounted filesystems on Posix hosts, like Samba shares or
    NFS-mounted Windows volumes, might be case-insensitive.  Can we
@@ -2248,65 +2251,33 @@ internal_delete_file (Lisp_Object filename)
 static bool
 file_name_case_insensitive_p (const char *filename)
 {
-#ifdef _PC_CASE_INSENSITIVE
+#ifdef DOS_NT
+  return 1;
+#elif defined CYGWIN
+/* As of Cygwin-2.6.1, pathconf supports _PC_CASE_INSENSITIVE.  */
+# ifdef _PC_CASE_INSENSITIVE
   int res = pathconf (filename, _PC_CASE_INSENSITIVE);
-  if (0 < res)
-    return true;
-  if (res == 0 || errno != EINVAL)
-    return false;
-#elif defined _PC_CASE_SENSITIVE
-  int res = pathconf (filename, _PC_CASE_SENSITIVE);
-  if (res == 0)
-    return true;
-  if (0 < res || errno != EINVAL)
-    return false;
-#endif
-
-#ifdef DARWIN_OS
-  /* It is not clear whether this section is needed.  For now, rely on
-     pathconf and skip this section.  If pathconf does not work,
-     please recompile Emacs with -DDARWIN_OS_CASE_SENSITIVE_FIXME=1 or
-     -DDARWIN_OS_CASE_SENSITIVE_FIXME=2, and file a bug report saying
-     whether this fixed your problem.  */
-# ifndef DARWIN_OS_CASE_SENSITIVE_FIXME
-  int DARWIN_OS_CASE_SENSITIVE_FIXME = 0;
+  if (res < 0)
+    return 1;
+  return res > 0;
+# else
+  return 1;
 # endif
+#elif defined DARWIN_OS
+  /* The following is based on
+     http://lists.apple.com/archives/darwin-dev/2007/Apr/msg00010.html.  */
+  struct attrlist alist;
+  unsigned char buffer[sizeof (vol_capabilities_attr_t) + sizeof (size_t)];
 
-  if (DARWIN_OS_CASE_SENSITIVE_FIXME == 1)
-    {
-      /* This is based on developer.apple.com's getattrlist man page.  */
-      struct attrlist alist = {.volattr = ATTR_VOL_CAPABILITIES};
-      struct vol_capabilities_attr_t vcaps;
-      if (getattrlist (filename, &alist, &vcaps, sizeof vcaps, 0) == 0)
-	{
-	  if (vcaps.valid[VOL_CAPABILITIES_FORMAT] & VOL_CAP_FMT_CASE_SENSITIVE)
-	    return ! (vcaps.capabilities[VOL_CAPABILITIES_FORMAT]
-		      & VOL_CAP_FMT_CASE_SENSITIVE);
-	}
-      else if (errno != EINVAL)
-	return false;
-    }
-  else if (DARWIN_OS_CASE_SENSITIVE_FIXME == 2)
-    {
-      /* The following is based on
-	 http://lists.apple.com/archives/darwin-dev/2007/Apr/msg00010.html.  */
-      struct attrlist alist;
-      unsigned char buffer[sizeof (vol_capabilities_attr_t) + sizeof (size_t)];
-
-      memset (&alist, 0, sizeof (alist));
-      alist.volattr = ATTR_VOL_CAPABILITIES;
-      if (getattrlist (filename, &alist, buffer, sizeof (buffer), 0)
-	  || !(alist.volattr & ATTR_VOL_CAPABILITIES))
-	return 0;
-      vol_capabilities_attr_t *vcaps = buffer;
-      return !(vcaps->capabilities[0] & VOL_CAP_FMT_CASE_SENSITIVE);
-    }
-#endif
-
-#if defined CYGWIN || defined DOS_NT
-  return true;
+  memset (&alist, 0, sizeof (alist));
+  alist.volattr = ATTR_VOL_CAPABILITIES;
+  if (getattrlist (filename, &alist, buffer, sizeof (buffer), 0)
+      || !(alist.volattr & ATTR_VOL_CAPABILITIES))
+    return 0;
+  vol_capabilities_attr_t *vcaps = buffer;
+  return !(vcaps->capabilities[0] & VOL_CAP_FMT_CASE_SENSITIVE);
 #else
-  return false;
+  return 0;
 #endif
 }
 
@@ -2378,7 +2349,7 @@ This is what happens in interactive use with M-x.  */)
   /* If the filesystem is case-insensitive and the file names are
      identical but for the case, don't ask for confirmation: they
      simply want to change the letter-case of the file name.  */
-  if ((! file_name_case_insensitive_p (SSDATA (encoded_file))
+  if ((!(file_name_case_insensitive_p (SSDATA (encoded_file)))
        || NILP (Fstring_equal (Fdowncase (file), Fdowncase (newname))))
       && ((NILP (ok_if_already_exists) || INTEGERP (ok_if_already_exists))))
     barf_or_query_if_file_exists (newname, false, "rename to it",