Male pattern baldness (MPB) and prostate cancer are common in American males; however, MPB is clinically observable decades earlier. Aging, androgens, and heritability are risk factors for both conditions. We prospectively studied the association between MPB and clinical prostate cancer in a cohort representative of the United States male population. A total of 4,421 men 25-75 years old without a history of prostate cancer were examined for baldness in the Epidemiologic Follow-up Study of the first National Health and Nutrition Examination Survey. Participants were followed from baseline (1971-1974) through 1992. Incident cases of prostate cancer were identified by interviews, medical records, and death certificates. Age-standardized incidence rates and proportional hazards models were used to examine the association between MPB and clinical prostate cancer. Prostate cancer was diagnosed in 214 subjects over 17-21 years of follow-up. The age-standardized incidence of prostate cancer was greater among men with baldness at baseline (17.5 versus 12.5 per 10,000 person-years). The adjusted relative risk for prostate cancer among men with baldness was 1.50 (95% confidence interval, 1.12-2.00) and was similar regardless of the severity of baldness at baseline and was independent of other risk factors, including race and age. MPB seems to be a risk factor for clinical prostate cancer.