We have attempted to clarify the relationships of the QTc interval to alpha and beta sympathetic, as well as parasympathetic function tests including spectral analyses of the R-R interval and systolic blood pressure. The QTc intervals were estimated in 76 diabetic patients and 76 age-matched healthy controls whose R-R intervals were comparable to those of the diabetic patients. We also investigated the relationships of the QTc interval to various clinical features of diabetes mellitus and to autonomic function tests. The QTc interval in diabetic patients was significantly greater than that in healthy controls. The QTc intervals were unrelated to diabetic control, retinopathy and nephropathy, but were prolonged in patients with a long duration of diabetes as compared to those with a short disease duration. There were significant correlations between the QTc interval and orthostatic hypotension, R-R variation and the Valsalva maneuver. Significant correlations were observed between the QTc interval and both the low and the high frequency component of spectral analysis of the R-R interval, whereas no correlations were found with spectral analysis of systolic blood pressure. An abnormal QTc interval is an indicator of cardiac sympathetic and parasympathetic nervous dysfunction, but not vasomotor dysfunction.