Evidence for vagal denervation of the heart as a feature of diabetic autonomic neuropathy has been obtained by monitoring beat-to-beat variation in heart rate. Nine diabetics with autonomic neuropathy were assessed; each showed a marked reduction or absence of beat-to-beat variation in comparison with controls. Beat-to-beat variation in normal subjects is abolished by parasympathetic blockade but unaffected by sympathetic blockade. These findings suggest that spontaneous vagal denervation of the heart was present in the cases studied. Measurement of beat-to-beat variation provides a simple test whereby cases of autonomic neuropathy can be screened for cardiac involvement.