A case-control study was performed to estimate the relative risk of developing prostate cancer for men with a positive family history. Extensive cancer pedigrees were obtained on 691 men with prostate cancer and 640 spouse controls. Fifteen percent of the cases but only 8% of the controls had a father or brother affected with prostate cancer (P less than .001). Men with a father or brother affected were twice as likely to develop prostate cancer as men with no relatives affected. In addition, there was a trend of increasing risk with increasing number of affected family members such that men with two or three first degree relatives affected had a five and 11-fold increased risk of developing prostate cancer. Recognizing that 9-10% of U.S. men will develop prostate cancer in their lifetime, men with a family history of prostate cancer should be advised of their significantly increased prostate cancer risk and should undergo appropriate screening measures for this disease.