diff --git a/scripts/latency-histogram b/scripts/latency-histogram
index 014b63f582f..409ed5ca144 100755
--- a/scripts/latency-histogram
+++ b/scripts/latency-histogram
@@ -111,6 +111,25 @@ proc which_exe {name} {
   return -code error "$name: executable not found"
 } ;# which_exe
 
+proc check_rt_privileges {} {
+  # Warn if rtapi reports no realtime ('halcmd getrt'), usually a missing
+  # 'sudo make setuid'.  LINUXCNC_NO_RT_WARN suppresses it on dev boxes.
+  if {[info exists ::env(LINUXCNC_NO_RT_WARN)]} return
+  set out ""
+  catch {exec $::LH(prog,halcmd) getrt 2>@1} out
+  if {![string match {*No realtime available*} $out]} return
+  set msg "Warning: realtime is not available."
+  append msg "\nThreads get no SCHED_FIFO priority or locked memory,"
+  append msg "\nso the latency shown will be far worse than reality."
+  append msg "\nOn a run-in-place build, run 'sudo make setuid' (or 'sudo make setcap') in src/."
+  puts stderr $msg
+  if {$::LH(use_x)} {
+    package require Tk
+    tk_messageBox -parent . -icon warning -type ok \
+        -title "Latency Warning" -message $msg
+  }
+} ;# check_rt_privileges
+
 proc program_check {plist} {
   foreach prog $plist {
     if [catch {
@@ -944,6 +963,7 @@ proc windowToFile { win } {
 if ![info exists ::LH(start)] {
   set_defaults
   config
+  check_rt_privileges
   progress "Loading packages"
   load_packages
   signal trap SIGINT finish
diff --git a/scripts/latency-test b/scripts/latency-test
index 63b34c0034d..8da38e8e1ba 100755
--- a/scripts/latency-test
+++ b/scripts/latency-test
@@ -16,6 +16,20 @@ kversion=$(uname -v)
 calc() { awk "BEGIN { print ($1); }" < /dev/null; }
 icalc() { awk "BEGIN { printf \"%.0f\n\", ($1); }" < /dev/null; }
 
+# Warn if rtapi reports no realtime ('halcmd getrt'), usually a missing
+# 'sudo make setuid'.  LINUXCNC_NO_RT_WARN suppresses it on dev boxes.
+check_rt_privileges () {
+    [ -n "$LINUXCNC_NO_RT_WARN" ] && return 0
+    case $(halcmd getrt 2>&1) in
+        *"No realtime available"*) ;;
+        *) return 0 ;;
+    esac
+    echo "Warning: realtime is not available: threads get no SCHED_FIFO priority" >&2
+    echo "         or locked memory, so the latency shown will be far worse than" >&2
+    echo "         reality.  On a run-in-place build, run 'sudo make setuid'" >&2
+    echo "         (or 'sudo make setcap') in src/." >&2
+}
+
 parse_time () {
     case $1 in
     -)   echo "0" ;;
@@ -170,4 +184,6 @@ L=\$(($SIGNALS
 echo \$L > "$HOME"/.latency
 EOF
 
+check_rt_privileges
+
 halrun lat.hal