Seventy-three diabetics (62 males and 11 females) who complained of symptoms suggestive of autonomic neuropathy were followed prospectively for up to five years. Thirty patients presented with impotence alone, while the other 43 presented with one or more of the following: postural hypotension, intermittent diarrhoea, hypoglycaemic unawareness, sweating abnormalities and gastric fullness. Most subjects with impotence alone had normal autonomic function tests (responses to the Valsalva manoeuvre and sustained handgrip) whereas the majority with other symptoms had abnormal tests. Twenty-six subjects (20 males and six females) died during the follow-up period. Of the 33 with initially normal autonomic function tests, five (15 per cent) died, whereas of the 40 with initially abnormal tests, 21 (53 per cent) died. Diabetics with symptoms of autonomic neuropathy and abnormal autonomic function tests, had a calculated mortality rate after two-and-a-half years of 44 per cent and after five years of 56 per cent. Half the deaths in those with abnormal tests were from renal failure, and the remainder were either sudden and unexpected, or from other causes which may have been associated with the autonomic neuropathy. Autonomic function testing repeated during the follow-up period showed that some normal tests later became abnormal, but once tests were abnormal, they usually remained abnormal. A number of subjects with impotence alone developed other features of autonomic neuropathy and abnormal tests during the follow up period. Symptoms of autonomic neuropathy, particularly postural hypotension, gastric symptoms and hypoglycaemic unawareness, together with abnormal autonomic function tests, carry a very poor prognosis. Diarrhoea and importence, on their own, cannot be relied on as symptoms of autonomic neuropathy. Autonomic function testing using simple cardiovascular reflexes give a good guide to the prognosis of diabetic autonomic neuropathy.