In order to investigate the safety and efficacy of sildenafil prescribed in primary care, a post-marketing surveillance study was undertaken. A total of 651 men with erectile dysfunction (ED) were enrolled from 31 family physicians in Korea from December 1999 to July 2002. Patients were regularly followed up to ascertain the safety and efficacy of sildenafil. Of the 651 patients enrolled, 572 (87.9%) returned for safety evaluation and efficacy assessment. In all, 458 (80.1%) of 572 patients reported improved erectile function with sildenafil. Hypertension, diabetes and low-dose sildenafil were associated with poor efficacy. A total of 71 adverse events were reported among 56 patients (8.6%), with the most frequent being hot flushes (5.6%), followed by headache (2.6%), palpitation (1.0%), anxiety (0.5%) and elevated ALT (0.5%). Only six patients (1.0%) discontinued sildenafil as a direct result of adverse events. These results suggest that sildenafil prescribed by primary care physicians was well tolerated and improved erectile function in patients with ED.