The natural history of erectile impotence in diabetic men has been defined in a 5-year prospective study of 466 patients initially aged 20-59 years. Of the 275 who were originally potent, 78 (28%) have become impotent. Five features present at first interview were found to be independently predictive of the subsequent development of impotence; age (p less than 0.0001), alcohol intake (p less than 0.0001), initial glycaemic control (p = 0.03), intermittent claudication (p = 0.04) and retinopathy (p = 0.05). The development of impotence was also significantly associated with the appearance of neuropathic symptoms (p = 0.003) and poor glycaemic control in the intervening 5 years (p = 0.01). Only 11 out of 128 (9%) of those originally impotent regained potency; they were young, had short duration of diabetes, and often features of psychogenic impotence. Those with impotence originally but no clinically apparent micro/macrovascular or neuropathic diabetic complications developed retinopathy (p = 0.001) and neuropathy (p = 0.01) more frequently than their comparable potent counterparts. It is concluded that diabetic impotence rarely reverses, that it is strongly associated with neuropathic and vascular complications of diabetes, and that moderation of alcohol consumption and improvement of glycaemic control are possible preventative factors.