The objective of this study was to determine the effects of oral phentolamine, administered before sleep, on nocturnal penile erectile activity of men with mild to moderate erectile dysfunction (ED). We studied five patients with mild to moderate ED (mean age 34.8 +/- 8.13 and mean duration of ED 31.8 +/- 23.5 months), in a double-blind, placebo-controlled, crossover study. All patients received oral phentolamine (Vasomax) at a dose of 40 mg and placebo for three consecutive nights respectively and were submitted to nocturnal penile tumescence and rigidity monitoring (NPTR) with the Rigiscan device. NPTR parameters of the two 3-night recordings were evaluated and compared. Administration of oral phentolamine before sleep was associated with a statistically significant increase in the number of erectile events with rigidity > or = 60% lasting > or = 10 min (P = 0.02), as well as the rigidity activity units (RAU) value per hour sleep, both at the base (P = 0.023) and the tip of the penis (P = 0.019). The number of events as measured by Rigiscan software (20% change in circumference), as well as tumescence activity units (TAU)/h values did not show any statistical difference. No adverse effects were recorded. It is concluded that oral phentolamine administered before sleep enhanced NPTR parameters associated with the quality of the erectile events. Such results provide a pathway for the development of a prevention strategy for ED. Future studies will elucidate whether vasoactive agents taken on a regular basis before sleep, can prevent ED in men at risk, protecting also minimally and moderately impotent patients to become moderately and severely impotent respectively.