Monitoring of nocturnal penile tumescence (NPT) is an objective method for determining the erectile status of patients who complain of impotence. The characteristics of 11 diabetic patients with abnormal NPT, 11 patients with normal NPT and no medical disorders and 11 normal men were presented. Several sociodemographic characteristics of the 1st 100 patients undergoing an NPT evaluation as part of a screen for patients seeking implantation of a penile prosthesis were described. NPT monitoring should be a component of the differential diagnosis procedure for every patient who complains of impotence and it provides a tool for the concentrated study of the mechanisms of erection and impotence.