diff --git a/collects/scribblings/reference/mpairs.scrbl b/collects/scribblings/reference/mpairs.scrbl
index 4d6cc6a0fa..952f231c7f 100644
--- a/collects/scribblings/reference/mpairs.scrbl
+++ b/collects/scribblings/reference/mpairs.scrbl
@@ -95,6 +95,15 @@ Like @scheme[list-tail], but for @tech{mutable lists}.}
 
 Like @scheme[append], but for @tech{mutable lists}.}
 
+@defproc*[([(mappend! [mlst mlist?] ...) mlist?]
+           [(mappend! [mlst mlist?] ... [v any/c]) any/c])]{
+
+The @scheme[mappend!] procedure appends the given lists by mutating the tail of
+each to refer to the next, using @scheme[set-mcdr!]. Empty lists are silently
+ignored; in particular, the result of calling @scheme[mappend!] with one or
+more empty lists is the same as the result of the call with the empty lists
+removed from the set of arguments.}
+
 
 @defproc[(mreverse [mlst mlist?]) mlist?]{