Twenty four men with insulin dependent diabetes and different degrees of autonomic neuropathy were studied to establish the response of the QT interval to various heart rates. Nine men with autonomic neuropathy had a longer QT interval than 13 healthy individuals and 15 patients who had diabetes without, or with only mild, autonomic neuropathy. Those with autonomic neuropathy also had a proportionally greater lengthening of the QT interval for a given increase in RR interval. The results of this study suggest a basis for the finding that sudden death is more common in patients with diabetic autonomic neuropathy.