PURPOSE: Sildenafil citrate(Viagra(R)) has been accepted as a primary treatment of erectile dysfunction(ED). We performed this study to assess the determinants of sildenafil failure and the status of secondary treatment in sildenafil failure patients. MATERIALS AND METHODS: We reviewed the medical records of 1,023 erectile dysfunction patients who were prescribed sildenafil from September 2001 to June 2003. A specially designed sildenafil failure questionnaire was asked in 236 patients of sildenafil failure(Group I) and 127 of 787 patients with sildenafil success as a control(Group II). RESULTS: Sildenafil failure rate was 23%(236/1023). In Group I, the etiology of ED was psychogenic in 14%, organic in 36%, mixed in 32% and un-classified in 18%. Number of total medication(more than 6 times) was 47% in Group I and 61.4% in Group II. Presence of sexual stimulation after medication was 47% in Group I and 61.4% in Group II. There was no difference in time to intercourse after medication and interval of meal to medication between both groups(p>0.05). The secondary treatment in Group I was intracavernous injection(11.3%), vacuum device(7.2%), penile prosthesis(1.3%) and 81.2% of no further treatment. CONCLUSIONS: Although precise education of taking sildenafil may enhance the response rate, the best predictors for long-term use of sildenafil are the severity and cause of ED and the effectiveness of sildenafil. Due to the high dropout rate of patients who suffer sildenafil failure, precise public education and secondary treatment options should be recommended.