diff --git a/bin/ps/ps.c b/bin/ps/ps.c
index dcb701821f53..6455ea66202d 100644
--- a/bin/ps/ps.c
+++ b/bin/ps/ps.c
@@ -55,6 +55,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/stat.h>
 #include <sys/ioctl.h>
 #include <sys/sysctl.h>
+#include <sys/mount.h>
 
 #include <ctype.h>
 #include <err.h>
@@ -128,6 +129,7 @@ struct listinfo {
 	} l;
 };
 
+static int	 check_procfs(void);
 static int	 addelem_gid(struct listinfo *, const char *);
 static int	 addelem_pid(struct listinfo *, const char *);
 static int	 addelem_tty(struct listinfo *, const char *);
@@ -401,6 +403,14 @@ main(int argc, char *argv[])
 	argc -= optind;
 	argv += optind;
 
+	/*
+	 * If the user specified ps -e then they want a copy of the process
+	 * environment kvm_getenvv(3) attempts to open /proc/<pid>/mem.
+	 * Check to make sure that procfs is mounted on /proc, otherwise
+	 * print a warning informing the user that output will be incomplete.
+	 */
+	if (needenv == 1 && check_procfs() == 0)
+		warnx("Process environment requires procfs(5)");
 	/*
 	 * If there arguments after processing all the options, attempt
 	 * to treat them as a list of process ids.
@@ -1166,6 +1176,21 @@ kludge_oldps_options(const char *optlist, char *origval, const char *nextarg)
 	return (newopts);
 }
 
+static int
+check_procfs(void)
+{
+	struct statfs *mntbuf;
+	size_t mntsize, i;
+
+	mntsize = getmntinfo(&mntbuf, MNT_NOWAIT);
+	for (i = 0; i < mntsize; i++)
+		if (strcmp(mntbuf[i].f_mntonname, "/proc") == 0 &&
+		    strcmp(mntbuf[i].f_fstypename, "procfs") == 0) {
+			return (1);
+		}
+	return (0);
+}
+
 static void
 usage(void)
 {