The effect of oral prostaglandin E1 (limaprost) on erectile function was studied in a double-blind placebo controlled trial. Fifty one patients who agreed to participate were examined for their subjective symptoms and nocturnal erection was recorded using an erectometer at the beginning of the study, after an initial 6 week period, and again after a second 6 week period. Patients were randomly assigned to a group which received placebo followed by limaprost or to a group which received limaprost followed by placebo. Ten cases dropped out. In the remaining forty one patients, NPT during the limaprost phase was significantly different from that during the placebo phase. Patients with the history of diabetes mellitus, hypertension, or pelvic surgery showed relatively poor responses to oral prostaglandin E1. Oral prostaglandin E1 achieved 42.9% effectiveness in the psychogenic impotence, and this effectiveness is significantly higher than that of placebo. Oral prostaglandin E1 was suggested as an additional or alternative therapy in the management of psychogenic impotence. Psychogenic impotent who didn't respond to sex therapy and patients with slight organic causes would seem to benefit from oral prostaglandin treatment.