diff --git a/misc.c b/misc.c
index e7b32c9..c9ec29f 100644
--- a/misc.c
+++ b/misc.c
@@ -29,7 +29,7 @@
 #include <grp.h>
 #endif
 
-__RCSID("$MirOS: src/bin/mksh/misc.c,v 1.139 2010/07/04 17:33:55 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/misc.c,v 1.140 2010/07/13 13:07:56 tg Exp $");
 
 unsigned char chtypes[UCHAR_MAX + 1];	/* type bits for unsigned char */
 
@@ -329,6 +329,12 @@ parse_args(const char **argv,
 				break;
 			}
 			i = option(go.optarg);
+			if ((enum sh_flag)i == FARC4RANDOM) {
+				warningf(true, "Do not use set ±o arc4random,"
+				    " it will be removed in the next version"
+				    " of mksh!");
+				return (0);
+			}
 			if ((i != (size_t)-1) && set == Flag(i))
 				/* Don't check the context if the flag
 				 * isn't changing - makes "set -o interactive"
diff --git a/mksh.1 b/mksh.1
index 92c09c1..e3eb68a 100644
--- a/mksh.1
+++ b/mksh.1
@@ -1,4 +1,4 @@
-.\" $MirOS: src/bin/mksh/mksh.1,v 1.228 2010/07/11 11:17:31 tg Exp $
+.\" $MirOS: src/bin/mksh/mksh.1,v 1.229 2010/07/13 13:07:56 tg Exp $
 .\" $OpenBSD: ksh.1,v 1.134 2010/05/10 21:04:54 jmc Exp $
 .\"-
 .\" Copyright © 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010
@@ -71,7 +71,7 @@
 .\" with -mandoc, it might implement .Mx itself, but we want to
 .\" use our own definition. And .Dd must come *first*, always.
 .\"
-.Dd $Mdocdate: July 11 2010 $
+.Dd $Mdocdate: July 13 2010 $
 .\"
 .\" Check which macro package we use
 .\"
@@ -3852,6 +3852,11 @@ during file name generation.
 Print commands and parameter assignments when they are executed, preceded by
 the value of
 .Ev PS4 .
+.It Ic arc4random
+Deprecated, will be removed in
+.Nm
+R40.
+Do not use, emits a warning to stderr.
 .It Ic bgnice
 Background jobs are run with lower priority.
 .It Ic braceexpand
diff --git a/sh_flags.h b/sh_flags.h
index 5a9e02c..aa5481e 100644
--- a/sh_flags.h
+++ b/sh_flags.h
@@ -1,5 +1,5 @@
 #if defined(SHFLAGS_DEFNS)
-__RCSID("$MirOS: src/bin/mksh/sh_flags.h,v 1.6 2010/07/04 17:33:58 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/sh_flags.h,v 1.7 2010/07/13 13:07:58 tg Exp $");
 #define FN(sname,cname,ochar,flags)	/* nothing */
 #elif defined(SHFLAGS_ENUMS)
 #define FN(sname,cname,ochar,flags)	cname,
@@ -21,6 +21,9 @@ __RCSID("$MirOS: src/bin/mksh/sh_flags.h,v 1.6 2010/07/04 17:33:58 tg Exp $");
 /* -a	all new parameters are created with the export attribute */
 F0("allexport", FEXPORT, 'a', OF_ANY)
 
+/* ./.	backwards compat: dummy, emits a warning */
+FN("arc4random", FARC4RANDOM, 0, OF_ANY)
+
 #if HAVE_NICE
 /* ./.	bgnice */
 FN("bgnice", FBGNICE, 0, OF_ANY)