Long-term efficacy and safety of sildenafil was assessed in 1008 patients with erectile dysfunction (ED) enrolled in four flexible-dose (25 - 100 mg), open-label, 36- or 52-week extension studies. After 36 and 52 weeks, 92% and 89% of patients felt that treatment with sildenafil had improved their erections. Responses to a Sexual Function Questionnaire indicated that 52 weeks of sildenafil treatment resulted in clinically significant improvements in the duration and firmness of erections, overall satisfaction with sex life, and the frequency of stimulated erections. Commonly reported adverse events (AEs) were headache, flushing, dyspepsia, and rhinitis, which were generally mild to moderate. Reports of abnormal vision were consistent with previous clinical trials. The occurrence of treatment-related cardiovascular AEs, such as hypertension, tachycardia, and palpitation, was <1%. Discontinuations due to treatment-related AEs were low (2%). Long-term therapy does not diminish the efficacy of sildenafil in patients with ED and remains well tolerated.