Heart rate variability (HRV) during deep breathing was studied with a neonatal heart monitor in 143 control subjects and 218 patients with diabetes (102 with IDDM and 116 with NIDDM). In the control group HRV decreased after age 20 by 4-5 beats per decade (from 29.7 +/- 5.8 beats at age 20-29 to 11.8 +/- 5.4 beats at age 60+). In all age groups HRV in IDDM was lower than in the controls, and both age and duration of diabetes played a role in the decrease of HRV (from 21.5 +/- 5.3 beats at age 20-29 to 6.3 +/- 5.4 at age 60+). In NIDDM aging seemed to play a less important role, and the influence of the duration of the disease was not statistically significant. In both groups of patients the frequency of HRV below the 2.5th percentile was 82% in those with symptoms and/or signs of autonomic neuropathy, 64% in patients with peripheral neuropathy only, and 36% in those who had no obvious signs or symptoms of neuropathy. Interindividual variability was pronounced, and age and duration of the disease together accounted for only 36% of the observed differences between IDDM and the controls. Determination of HRV with a standard neonatal heart monitor presents an easy, simple, and nonstressful test of cardiac autonomic neuropathy. The norms of the test are age related.