Erectile dysfunction is a common complication of spinal cord injury. This double-blind, placebo-controlled, two-way crossover study assessed the efficacy and safety of oral sildenafil in men with erectile dysfunction caused by traumatic spinal cord injury. A total of 178 men (mean age, 38 years) received placebo or sildenafil 1 hour before sexual activity for 6 weeks; after a 2-week washout period, the men received the alternate treatment for 6 weeks. The 50-mg starting dose could be adjusted to 100 or 25 mg based on efficacy and tolerability. Efficacy was assessed by using global efficacy questions, the International Index of Erectile Function (IIEF), and a patient log of erectile activity. Of 143 men with residual erectile function at baseline, 111 (78%) reported improved erections and preferred sildenafil to placebo. For all men (including those who reported no residual erectile function at baseline), 127 of 168 (76%) reported improved erections and preferred sildenafil to placebo. For all men, 132 of 166 (80%) reported that sildenafil improved sexual intercourse compared with 17 of 166 men (10%) reporting improvement with placebo. IIEF questions assessing the ability to achieve and maintain erections and satisfaction with sexual intercourse demonstrated significant improvement with sildenafil. Sildenafil was well tolerated, with a low rate of discontinuation because of treatment-related adverse events (2% vs 1% for placebo). Oral sildenafil is an effective and well-tolerated treatment for erectile dysfunction caused by spinal cord injury.