The prevalence and correlates of erectile dysfunction (ED) in developing countries are largely unknown. Our objectives were to determine the prevalence and associated factors of ED in three countries (Pakistan, Egypt, Nigeria) that represent very different cultures. Men 35-70y of age seeking primary medical care answered a structured questionnaire adapted to reflect local cultures. Degree of ED was categorized as 'none,' 'mild,' 'moderate,' or 'complete.' The age-adjusted prevalence rates of ED among men attending primary care clinics was 57.4% in Nigeria, 63.6% in Egypt, and 80.8% in Pakistan. Older age, diabetes, peptic ulcers, prostate disease, depression-related symptoms, and caffeine consumption were independently associated with increased prevalence of ED, whereas being moderately active to very active at work (hard physical labor) and during leisure time (strenuous exercise) was associated with half the prevalence of moderate-to-complete ED. Our multicultural study demonstrates that in every country studied, high proportions of men older than age 35 have some degree of ED (57-81%). Both severity and prevalence increase consistently with age. Factors associated with ED are similar, but their distribution differs across countries.