diff --git a/scripts/latency-histogram b/scripts/latency-histogram
index 245a2c386ee..3a9001ead66 100755
--- a/scripts/latency-histogram
+++ b/scripts/latency-histogram
@@ -111,6 +111,24 @@ proc which_exe {name} {
   return -code error "$name: executable not found"
 } ;# which_exe
 
+proc check_rt_privileges {} {
+  # Ask the realtime layer whether realtime is actually available, via
+  # 'realtime verify' (exits 0 when realtime is up, non-zero otherwise,
+  # usually a missing 'sudo make setuid' or 'sudo make setcap').
+  if {[catch {exec linuxcnc_var REALTIME} rt]} return ;# cannot locate, stay quiet
+  if {![catch {exec $rt verify 2>@1}]} return         ;# realtime available
+  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 $plist {
     if [catch {
@@ -624,6 +642,7 @@ proc start_collection {} {
     }
   }
   hal start
+  check_rt_privileges
   if {!$::LH(legacy)} {
     # Attach the Tcl hal_stream binding (added in halsh.c) to each
     # thread's latencybinstream FIFO. The handle stays open for the