Purpose: This study was done to assess the therapeutic effect of trazodone, an antidepressant agent in combination with sildenafil in patients with erectile dysfunction (ED) and decreased libido with normal serum testosterone level. Materials and Methods: From March 2005 to February 2006, 90 patients with ED complaining of decreased libido but whose serum testosterone level was within normal range were screened and enrolled in this study. The subjects were randomly treated by sildenafil only (group 1) and combination of sildenafil with trazodone (group 2). Patients received trazodone for at least 3 consecutive months, with daily doses starting at 50 mg. By intention-to-treat basis, the period of study was extended by 9 month. The intention-to-treat population included 39 sildenafil recipients and 35 sildenafil with trazodone recipients. The patients were asked to make up two questions, Q1: 'Did you experience improvement of erection after treatment?' and Q2: 'Did you experience improvement of libido after treatment?' and the change of International index of erectile function (IIEF) scores before and after the treatment were analyzed to assess the efficacy of treatment. Results: After 3 month treatment period, 70% of group 1 and group 2 answered 'yes' to Q1. Twenty seven percents of group 1 and 51% of group 2 answered 'yes' to Q2 and 20% of group 1 and 24% of group 2 answered 'yes' to both Q1 and Q2. In the intention-to-treat population, seventy-two percents of group 1 and 71% of group 2 answered 'yes' to Q1 and 28% of group 1 and 52% of group 2 answered 'yes' to Q2, 21% of group 1 and 26% of group 2 answered 'yes' to both Q1 and Q2. The change of IIEF scores before and after the treatment at 3 month and 9 month showed similar results. Both group 1 and group 2 showed significant improvement on scores of IIEF-5 and question 15 of IIEF after treatment (p<0.01). Meanwhile, only group 2 showed significant improvement on question 11 and 13 of IIEF after the treatment compared to group 1 (p<0.05). Two patients with high dose of trazodone complained of headache and drowsiness, but the symptoms disappeared by dose reduction. Conclusions: The combination therapy of trazodone with sildenafil increased libido and sexual satisfaction on patients with ED. Therefore, combination of trazodone with sildenafil might be considered in ED patients of decreased libido with normal serum testosterone level rather than sildenafil monotherapy.