diff --git a/syntax/ats.vim b/syntax/ats.vim
index 453dec6..b4d6c9b 100644
--- a/syntax/ats.vim
+++ b/syntax/ats.vim
@@ -62,7 +62,7 @@ endif
 
 " keyword highlighting
 syn keyword atsKeyword prefix postfix infix infixl infixr op nonfix addr
-syn keyword atsKeyword staload stadef sta dynload stacst assertloc
+syn keyword atsKeyword staload stadef sta dynload stacst assertloc macdef
 syn keyword atsCond if then else case sif when
 syn keyword atsRepeat while
 syn keyword atsException exception raise try