We studied the effects of sildena®l on nocturnal penile erections. We prospectively evaluated 36 patients with organic or psychogenic impotence and 5 normal, potent men. All patients completed 3 sessions of consecutive nights using the RigiScan Plus device. The ®rst two nights the patients were asked to take placebo before the session and to take 50 mg of sildena®l before the third session. In the organic impotence group the use of sildena®l induced a signi®cant improvement in time of rigidity 60 ± 100%, rigidity and tumescence activity unit values and rigidity and tumescence activity unit values per hour in the tip and base. In the psychogenic impotence group it caused signi®cant improvement only in rigidity activity unit per hour in the tip. In the potent men, changes were statistically insigni®cant. Sildena®l improves nocturnal penile erectile activity in organic impotence. Our study shows that phosphodiesterase inhibitors can improve penile erections not induced by sexual stimulation.