Autonomic nervous function was evaluated in 36 patients with insulin-dependent diabetes mellitus (IDDM), 39 patients with non-insulin-dependent diabetes mellitus (NIDDM) and 48 control subjects, all without clinically evident cardiovascular disease. Valsalva ratio and heart rate variation during deep breathing were lower in both diabetic groups than in the control group. Autonomic nervous function score (ANFS) was more abnormal in patients with IDDM than in control subjects, but was not significantly increased in patients with NIDDM. There was a negative correlation between ANFS and left ventricular diastolic filling evaluated by echocardiography or peak heart rate during exercise in both diabetic groups. There were no correlations between ANFS and left ventricular systolic function at rest or during exercise in any of the groups. In conclusion, autonomic nervous function was abnormal in middle-aged diabetic patients, and it was associated with impaired left ventricular diastolic filling at rest and decreased heart rate response to exercise, but not with left ventricular systolic function.