Cancer of the prostate is one of the most common cancers among males in North America. Although some causative factors have been suggested by several surveys, the etiology of this common cancer is poorly understood. In a case-control study of prostatic cancer in Greater Montreal, 21 of 140 patients with prostatic cancer (15.0%) gave a positive family history of the same cancer, as compared with two cases among 101 (2.0%) population-based controls. This indicates about an eightfold difference in occurrence of cancer of the prostate among first-degree family members of the case group with an odds ratio (OR) of 8.7 and 95% CI, 2.00–38.17. In this report we present the pedigrees of three families (two cases and one control) with four pathologically confirmed cases of cancer of the prostate in each family. This data suggest that a familial predisposition to prostatic cancer may become apparent in later decades of life. In these family aggregations, in addition to the genetic factors, environmental factors may also play an important role in the etiology of the same cancer among family members.