Diurnal blood pressure variation was studied by ambulatory 24-hour monitoring in patients with advanced chronic renal failure (n = 20), on chronic hemodialysis (n = 20), after renal transplantation (n = 21) and in matched control groups without renal disease. Nocturnal blood pressure reductions were significantly blunted in all patient groups as compared with the respective control groups. In almost none of the 61 controls did the mean values during nighttime (8 p.m.-8 a.m.) exceed the mean day time values (8 a.m.-8 p.m.). In 10 of the 61 renal patients blood pressure was higher during the night. In patients with chronic renal disease nocturnal blood pressure elevation may be diagnosed by ambulatory 24-hour monitoring. This may require adaptation of antihypertensive treatment.