The objectives of this study were to evaluate long-term safety and efficacy of phentolamine mesylate, an orally active, rapid-acting alpha-adrenergic receptor antagonist, for the treatment of men suffering from erectile dysfunction (ED). It was an open-label study involving more than 2000 patients. Men received phentolamine mesylate 40 mg or 80 mg (10 tablets/month) as needed for up to 13 months and self-assessed erectile performance using two validated questionnaires. Treatment with phentolamine mesylate was associated with increases in Erectile Function Domain score of the IIEF, successful vaginal penetrations, and in overall satisfaction. Most adverse events were mild or moderate in severity and consistent with the known pharmacodynamic properties of phentolamine. In conclusion, phentolamine mesylate is safe and effective in the long-term treatment of men with mild to moderate ED.