diff --git a/src/ca/cacIO.h b/src/ca/cacIO.h
index 41c7422c9..9ea926a92 100644
--- a/src/ca/cacIO.h
+++ b/src/ca/cacIO.h
@@ -55,6 +55,7 @@
 #include "tsDLList.h"
 #include "epicsMutex.h"
 #include "epicsGuard.h"
+#include "epicsThread.h"
 #include "epicsSingleton.h"
 
 #ifdef cacIOh_restore_epicsExportSharedSymbols