diff --git a/usr.sbin/makefs/makefs.8 b/usr.sbin/makefs/makefs.8
index 2ffa512a3b9..5c3d88b4ac1 100644
--- a/usr.sbin/makefs/makefs.8
+++ b/usr.sbin/makefs/makefs.8
@@ -113,7 +113,12 @@ Treat duplicate paths in an mtree manifest as warnings not error.
 Enable various levels of debugging, depending upon which bits are
 set in
 .Ar debug-mask .
-XXX: document these
+This option is intended for source debugging.
+.Ar debug-mask
+is a bit map defined in the header file
+.Ar makefs.h .
+See the source for usage, and look for defines starting with
+.Ar DEBUG_ .
 .It Fl F Ar mtree-specfile
 .Em This is almost certainly not the option you are looking for.
 To create an image from a list of files in an mtree format manifest,