Analysis of questionnaire replies from 292 diabetic men aged 20-59 revealed impotence in 23% (66), compared to 9% (7) of a sample of 81 non-diabetic outpatients. Further investigation of 45 impotent diabetic men and 45 matched non-impotent diabetic controls revealed greater psychological stress and poorer glycaemic control in the impotent group. There were no significant differences in the frequency of peripheral neuropathy, retinopathy, nephropathy or large vessel disease in the 2 groups. 20% (9) of the impotent diabetic group had definite autonomic neuropathy which was not present in any of the non-impotent group. The frequency of early autonomic damage was, however, similar in both groups. Impotence in diabetic men is associated with hyperglycaemia, stress and definite autonomic neuropathy. Our data suggest that men with early autonomic neuropathy are frequently potent.