To examine the relation between family history of breast cancer in a mother or sister and a man's risk of fatal prostate cancer, we analyzed data from a prospective mortality study of adult men in the United States. During 12 years of follow-up, there were 3,141 deaths from prostate cancer in a cohort of 480,802 men who were cancer-free at study entry in 1982. Results from Cox proportional hazards models, adjusted for other risk factors, showed a modest increased risk of fatal prostate cancer associated with a family history of breast cancer (in the absence of a family history of prostate cancer) [rate ratio (RR) = 1.16; 95% confidence interval (CI) = 1.01-1.33]. The association was stronger among men younger than 65 years of age whose relatives were diagnosed with breast cancer before age 50 years (RR = 1.65; 95% CI = 0.88-3.10) and among Jewish men (RR = 1.73; 95% CI = 1.00-2.97). The increased risks observed in these subgroups may reflect genetic alterations underlying familial clustering of prostate and breast cancer.