We have attempted to clarify the relationships of the QT c interval to α and β sympathetic, as well as parasympathetic function tests including spectral analyses of the R-R interval and systolic blood pressure. The QT c 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 QT c interval to various clinical features of diabetes mellitus and to autonomic function tests. The QT c interval in diabetic patients was significantly greater than that in healthy controls. The QT c 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 QT c interval and orthostatic hypotension, R-R variation and the Valsalva maneuver. Significant correlations were observed between the QT c 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 QT c interval is an indicator of cardiac sympathetic and parasympathetic nervous dysfunction, but not vasomotor dysfunction.