鈥� Nocturnal penile tumescence (NPT) was assessed in 30 impotent patients for the purpose of differential diagnosis. In 11 psychogenic cases, with several exceptions, NPT was found to be normal, with a marked discrepancy between the NPT findings and the patients' estimates of their daytime erectile capacity. In 19 organic patients, maximal NPT corresponded closely to and mirrored the patient's subjective estimate of his impaired waking performance. Statistically, the figures for frequency, degree, duration, and amount of NPT were found to be significantly and markedly greater in the psychogenic group. Statistical significance was maintained when the 11 psychogenic patients were closely age-matched with 11 of the 19 organic cases. The method is not 100% accurate; some 15% of impotent patients without organic findings have reduced NPT, several of whom were shown to be psychogenic, but others in this group may have undetected organic pathology. The NPT method is very promising, but beset by a number of methodological problems.