Partial or complete impotence is common in uremia. It is not clear whether the impotence is organic or psychogenic in nature and whether uremia itself or the state of chronic illness is responsible for it. We examined these questions, by psychiatric interviews and nocturnal penile tumescence (NPT), in 50 normal subjects, 48 patients with chronic uremia, including 23 patients treated with maintenance dialysis, and 22 patients with chronic illness and normal renal function. About 40 to 50% of patients with uremia, but not those with chronic illness and normal renal function, complained of erectile dysfunction and reported a significant decrease in frequency of intercourse. There were no significant differences between patients with uremia prior to initiation of therapy and those treated with maintenance hemodialysis. NPT declines after 40 years of age. In all age groups, NPT was significantly (P less than 0.01) lower in uremics than in normals or those with chronic illness. There was no correlation between erectile complaints, frequency of intercourse or NPT, and the presence or absence of depression. The frequency of intercourse correlated significantly (r = 0.68, P less than 0.01) with NPT in patients with uremia. Data indicate that 50% of male patients with uremia have partial or complete impotence, which is most probably organic in nature and is related to uremia or its metabolic or hormonal consequences rather than to the state of chronic illness.