This chapter describes the application of the genetic epidemiological approach to the study of human prostate cancer. We review the evidence for the familial clustering of prostate cancer and the Mendelian nature of this aggregation. The nature of this clustering is such that the closer genetically a man is to an affected relative and the greater number of relatives affected in a man's family, the greater his risk of prostate cancer. A complex segregation analysis of the 691 prostate cancer families showed that prostate cancer clustering can be explained by Mendelian inheritance of a rare autosomal gene producing prostate cancer at an early age. A model of inherited prostate cancer in the setting of multistep carcinogenesis is presented. The implications of these data for clinicians who diagnose and treat prostate cancer are also discussed.