Abstract Purpose: To assess efficacy of sildenafil citrate in treatment of erectile dysfunction: effect of type 2 diabetes. Materials and Methods: A total of 466 male patients with erectile dysfunction (ED) were enrolled in this study. Of them 382 were diabetic and 84 were non-diabetic. Patients were screened for ED using the erectile function domain of the International Index for Erectile Function (IIEF). Patients underwent routine laboratory investigations, in addition to total testosterone and prolactin assessment. To assess the effect of diabetes on efficacy of sildenafil, we compared the pre and post sildenafil responses to erectile function domain, Q3, Q4. Overall satisfaction and global efficacy question (GEQ) were also assessed. Results: Mean age ± S.D. was 53 ± 8.4 and 49.7 ± 10.6 years for patients with and without diabetes respectively. There were significant associations between increased severity of ED and longer duration, poor metabolic control and presence of more than one diabetes-related complication ( p < 0.05 for each). Differences were significant between pre and post sildenafil administration regarding erectile function domain, Q3, Q4 ( p < 0.05 for each). In the non-diabetic patients the GEQ and the overall satisfaction were significantly higher than in diabetics ( p < 0.05 for each). Global efficacy question was significantly low in patients with fair and poor metabolic control, longer duration of diabetes, and patients with diabetic complications ( p < 0.05 for each). Conclusions: Sildenafil is an effective treatment for diabetic patients with ED. Although the efficacy of sildenafil was negatively affected by factors as poor control and longer duration of diabetes and presence of more than one diabetes-related complication, however, the global efficacy and the overall patients’ satisfaction were high.