In order to assess the prevalence and associated factors for erectile dysfunction (ED) in primary care, a cross-sectional study was undertaken by questionnaire distributed to consecutive adult male attendees at 32 family practices. ED was assessed by the Korean five-item version of the International Index of Erectile Function (IIEF-5). In total, 3501 completed questionnaires were available for analysis. The prevalence of ED was severe (IIEF-5 score: 5-9) in 1.6% of cases, moderate (10-13) in 10.2%, mild (14-17) in 24.7%, and normal (18-25) in 63.4%. The prevalence of ED increased with age, lower educational status, heavy job-related physical activity, and lower income. ED prevalence was significantly higher in patients with chronic diseases such as diabetes, depression, and anxiety. These results suggest that the age-adjusted prevalence of ED among Korean men can be estimated as 32.2% (95% CI 30.6-33.7). Low socioeconomic status and several diseases such as diabetes, anxiety, and depression, as well as age, were associated with ED.