There is an extensive clinical literature on the erectile disorders of diabetic men but a paucity of controlled studies that have taken into account the effects of age, concurrent illnesses and medication on sexual function. This investigation was carried out on 40 diabetic men free from other illness or drugs that could affect sexual capacity and 40 age-matched healthy control subjects. Each subject and his female partner underwent semistructured interviews and the men had comprehensive medical evaluations and polygraphic assessment of sleep and nocturnal penile tumescence in the sleep laboratory during three nights. In comparison to control subjects, diabetic patients reported significant decreases in sexual desire, subjective arousal, erectile capacity, coital frequency and sexual satisfaction. The diabetic group also had significant decrements in duration of rapid eye movement sleep and in frequency, duration and degree of nocturnal penile tumescent episodes. There were no differences between Type 1 (insulin-dependent) and Type 2 (non-insulin dependent) diabetic patients in prevalence of sexual problems or in nocturnal tumescent measures. Significant relations were observed between lack of metabolic control, diabetic complications and impaired nocturnal tumescence. Sexually non-dysfunctional diabetic men had significant nocturnal penile tumescence abnormalities. Diabetic men without coital failures may have a subclinical impairment in erectile function which, although of not significant magnitude to interfere with penetration, is reflected in nocturnal penile tumescent measures. This result raises a note of caution in the interpretation of the nocturnal penile tumescence test for the differential diagnosis of diabetic erectile impotence.