diff --git a/src/support/FileName.cpp b/src/support/FileName.cpp
index 95880a0e79..05492f66d1 100644
--- a/src/support/FileName.cpp
+++ b/src/support/FileName.cpp
@@ -17,6 +17,7 @@
 #include "support/filetools.h"
 #include "support/lassert.h"
 #include "support/lstrings.h"
+#include "support/mutex.h"
 #include "support/qstring_helpers.h"
 #include "support/os.h"
 #include "support/Package.h"
@@ -910,13 +911,15 @@ string DocFileName::outputFileName(string const & path) const
 
 string DocFileName::mangledFileName(string const & dir) const
 {
-	// FIXME THREAD
 	// Concurrent access to these variables is possible.
 
 	// We need to make sure that every DocFileName instance for a given
 	// filename returns the same mangled name.
 	typedef map<string, string> MangledMap;
 	static MangledMap mangledNames;
+	static Mutex mangledMutex;
+	// this locks both access to mangledNames and counter below 
+	Mutex::Locker lock(&mangledMutex);
 	MangledMap::const_iterator const it = mangledNames.find(absFileName());
 	if (it != mangledNames.end())
 		return (*it).second;
@@ -942,7 +945,6 @@ string DocFileName::mangledFileName(string const & dir) const
 	// Add the extension back on
 	mname = support::changeExtension(mname, getExtension(name));
 
-	// FIXME THREAD
 	// Prepend a counter to the filename. This is necessary to make
 	// the mangled name unique.
 	static int counter = 0;