Objective: To determine the efficacy and safety of sildenafil citrate in the treatment of male erectile dysfunction. Data Sources: The MEDLINE, HealthSTAR, Current Contents, and Cochrane Library databases (January 1, 1995, through December 31, 2000); bibliographies of retrieved articles and review articles; conference proceedings abstracts; the Food and Drug Administration Web site; and the manufacturer. Study Selection: Trials were eligible if they included men with erectile dysfunction, compared sildenafil with control, were randomized, were of at least 7 days' duration, and assessed clinically relevant outcomes. Data Extraction: Two reviewers independently evaluated study quality and extracted data in a standardized fashion. Data Synthesis: Twenty-seven trials (6659 men) met the inclusion criteria. In results pooled from 14 parallel-group, flexible as-needed dosing trials, sildenafil was more likely than placebo to lead to successful sexual intercourse, with a higher percentage of successful intercourse attempts (57% vs 21%; weighted mean difference, 33.7; 95% confidence interval [CI], 29.2-38.2; 2283 men) and a greater percentage of men experiencing at least 1 intercourse success during treatment (83% vs 45%; relative benefit increase, 1.8; 95% CI, 1.7-1.9; 2205 men). In data pooled from 6 parallel-group, fixed-dose trials, efficacy appeared slightly greater at higher doses. Treatment response appeared to vary between patient subgroups, although relative to placebo, sildenafil significantly improved erectile function in all evaluated subgroups. In trials with parallel-group design and flexible dosing, men randomized to receive sildenafil were less likely than those receiving placebo to drop out for any reason and no more likely to drop out due to an adverse event or laboratory abnormality. Specific adverse events with sildenafil included flushing (12%), headache (11%), dyspepsia (5%), and visual disturbances (3%); all adverse events were significantly less likely to occur with placebo. Sildenafil was not significantly associated with serious cardiovascular events or death. Conclusions: Sildenafil improves erectile function and is generally well tolerated. Treatment response seems to vary between patient subgroups, although sildenafil has greater efficacy than placebo in all evaluated subgroups.