The authors conducted a study of nocturnal penile tumescence (NPT) in 35 diabetic men, aged 33 to 70, who complained of impotence and in 35 age-matched control subjects. EEGs and other measurements showed that the diabetic men as a group exhibited significant reductions in the total amount of NPT and in the amount and frequency of full erection, thus suggesting that impotence in this cohort was organogenic. Although NPT monitoring represents an advance over the less precise traditional procedures for the differential diagnosis of impotence, the authors stress the need for more research in this area.