We investigated the risk of prostate cancer in relation to a family history of prostate cancer in 58,279 men ages 55-69 years. We found 704 incident cases after 6.3 years of follow-up. Rate ratios and 95% confidence intervals for having an affected vs nonaffected father and brother were, respectively 1.44 (0.80-2.58) and 5.57 (1.61-19.26). We found no evidence for an increasing risk with an increasing percentage of affected family members. The associations we observed were stronger for cases diagnosed before age 70 compared with cases diagnosed after age 70 and for advanced compared with localized tumors.