diff --git a/includes/rts/Threads.h b/includes/rts/Threads.h
index 4bb480cf2b4f5721bf787a5e5d91f609edb277a1..7f016decb6b457dd56dccf04b964d48674f75423 100644
--- a/includes/rts/Threads.h
+++ b/includes/rts/Threads.h
@@ -42,7 +42,7 @@ StgRegTable * resumeThread  (void *);
 // Thread operations from Threads.c
 //
 int     cmp_thread                       (StgPtr tso1, StgPtr tso2);
-int     rts_getThreadId                  (StgPtr tso);
+long    rts_getThreadId                  (StgPtr tso);
 void    rts_enableThreadAllocationLimit  (StgPtr tso);
 void    rts_disableThreadAllocationLimit (StgPtr tso);
 
diff --git a/includes/rts/storage/TSO.h b/includes/rts/storage/TSO.h
index d706282796b321b2a684d58f0cb02cb299471133..3a488d97b5505f9224baecb2ffe8f61d3468e2d4 100644
--- a/includes/rts/storage/TSO.h
+++ b/includes/rts/storage/TSO.h
@@ -20,9 +20,9 @@ typedef struct {
  */
 
 /*
- * Thread IDs are 32 bits.
+ * Thread IDs are 64 bits.
  */
-typedef StgWord32 StgThreadID;
+typedef StgWord64 StgThreadID;
 
 #define tsoLocked(tso) ((tso)->flags & TSO_LOCKED)
 
diff --git a/rts/Threads.c b/rts/Threads.c
index 8b05dd7c50c26fa3e7beb712bdcfcedba6aea6b6..cce32ca2b38c8d6428ee9827d677701a2517cdea 100644
--- a/rts/Threads.c
+++ b/rts/Threads.c
@@ -161,7 +161,7 @@ cmp_thread(StgPtr tso1, StgPtr tso2)
  *
  * This is used in the implementation of Show for ThreadIds.
  * ------------------------------------------------------------------------ */
-int
+long
 rts_getThreadId(StgPtr tso)
 {
   return ((StgTSO *)tso)->id;
@@ -882,8 +882,8 @@ printThreadBlockage(StgTSO *tso)
     debugBelch("is blocked on an STM operation");
     break;
   default:
-    barf("printThreadBlockage: strange tso->why_blocked: %d for TSO %d (%p)",
-         tso->why_blocked, tso->id, tso);
+    barf("printThreadBlockage: strange tso->why_blocked: %d for TSO %ld (%p)",
+         tso->why_blocked, (long)tso->id, tso);
   }
 }