We assessed the hourly coefficient of variance of RR intervals (CVRR) in 29 subjects with non-insulin-dependent diabetes mellitus and 19 age-matched healthy subjects using an ambulatory 24-hour ECG monitoring system. We also evaluated CVRR during 100 beats in both groups in the supine posture at 07:00 and 10:00 in the morning. Hourly CVRR showed an overt diurnal variation in both groups especially in the daytime and at midnight, but was significantly lower (p less than 0.05) in diabetic patients. CVRR during resting 100 beats at 07:00 was significantly higher (p less than 0.05) than that at 10:00 in both groups. These findings indicate that HRV shows an obvious diurnal variation in both groups. As a steep and marked change occurs in the morning, careful consideration is necessary to assess the test performed for a short time span at random times throughout the day, particularly in the morning.