PURPOSE: We estimated the incidence of erectile dysfunction in men 40 to 69 years old at study entry during an average 8.8-year followup, and determined how risk varied with age, socioeconomic status and medical conditions. MATERIALS AND METHODS: Data from a randomly sampled population based longitudinal study of Massachusetts men were analyzed. A total of 1,709 men completed the baseline interview during 1987 to 1989 and 1,156 survivors completed followup from 1995 to 1997. The analysis sample consisted of 847 men without erectile dysfunction at baseline and with complete followup information. Erectile dysfunction was assessed by discriminant analysis of 13 questions from a self-administered sexual function questionnaire and a single global self-rating question. RESULTS: The crude incidence rate for erectile dysfunction was 25.9 cases per 1,000 man-years (95% confidence interval [CI] 22.5 to 29.9). The annual incidence rate increased with each decade of age and was 12.4 cases per 1,000 man-years (95% CI 9.0 to 16.9), 29.8 (24.0 to 37.0) and 46.4 (36.9 to 58.4) for men 40 to 49, 50 to 59 and 60 to 69 years old, respectively. The age adjusted risk of erectile dysfunction was higher for men with lower education, diabetes, heart disease and hypertension. Population projections for men 40 to 69 years old suggest that 17,781 new cases of erectile dysfunction in Massachusetts and 617,715 in the United States (white males only) are expected annually. CONCLUSIONS: Although prevalence estimates and cross-sectional correlates of erectile dysfunction have recently been established, incidence estimates were lacking. Incidence is necessary to assess risk, and plan treatment and prevention strategies. The risk of erectile dysfunction was about 26 cases per 1,000 men annually, and increased with age, lower education, diabetes, heart disease and hypertension.