Few studies on the pharmacological assessment or treatment of impotence have included controls. In a double-blind crossover study, 18 impotent men received either a 2 ml injection of a solution containing 30 mg papaverine and 1 mg phentolamine or a similar volume of normal saline into the corpus cavernosum. The immediate and delayed effects (4 weeks later) were assessed independently and the alternative injection then given. All injections with papaverine and phentolamine were followed by an immediate increase in penile length and rigidity, and 70% of these patients were able to have sexual intercourse for periods of 1 to 4 weeks. No change in penile length or rigidity occurred following the saline injection and only one patient showed some improvement in erection over the next 4 weeks. In the crossover study, no patient developed any increase in length or rigidity of the penis following injection of saline, and improvement in spontaneous erection occurred in only two cases. In contrast, all patients who received papaverine and phentolamine had an increase in penile length and rigidity, and 50% were able to have normal sexual intercourse over the next 4 weeks. This study confirms the value of a combination of vasoactive agents in the management of impotence, irrespective of its aetiology, and suggests that any placebo effect is minimal in this group of patients with considerable psychological overlay.