Objectives: To determine the frequency, severity and risk factors of erectile dysfunction in patients with diabetes mellitus. Material & Method: This study is descriptive analytical cross-sectional study that was carried out in medical wards, medical out patients department and Diabetic clinic of civil hospital Karachi on patients with established diagnosis of diabetes mellitus from 16-06-2007 to 15-12-2007. Three hundred ninety (n=390) adult male sexually active diabetic patients, fulfilling the selection criteria were enrolled for this study. The patients were interviewed and screened for erectile dysfunction by using the international index of Erectile Functoin-5(IIEF-5) questionnaire. Results: Mean age of the study population was 48.28 years (range 18-75 years). Out of three hundred ninety (390) patients interviewed, three hundred ten (n=310, 79.5%) reported having various degrees of erectile dysfunction. Mild (n=76, 24.5%), moderate in (n=80, 25.8 %) and severe (n= 154, 49.7.%). There was significant association between frequency & severity of erectile dysfunction with age, duration of diabetes, body mass index, hypertension, and use of anti hypertensive drugs, treatment modalities for diabetes control, smoking and number of cigarettes but not with duration of smoking. Conclusion: Erectile dysfunction is a very common condition in diabetic men and therefore should be routinely sought for by the clinicians. This study provides a quantitative estimate of the frequency of erectile dysfunction and its main risk factors in diabetic patients.