Increased risk of prostate cancer in men with a family history of the disease has been observed consistently in epidemiologic studies. However, most studies have been confined to white men; little is known about familial aggregation of prostate cancer in populations with unusually high incidence, such as African Americans, or in populations with low incidence, such as Asian-Americans. The authors report results from a population-based case-control study of prostate cancer among blacks, whites, and Asian-Americans in the United States and Canada. Controls were matched to cases on age (5-year groups), ethnicity (black, white, Chinese-American, Japanese-American), and region of residence (Los Angeles, San Francisco, Hawaii, Vancouver, Toronto). In the combined group of participants, 5% of controls and 13% of cases reported a father, brother, or son with prostate cancer. These prevalences were somewhat lower among Asian-Americans than among blacks or whites. A positive family history was associated with a statistically significant two- to threefold increase in risk in each of the three ethnic groups. The overall odds ratio associated with such a family history, adjusted for age and ethnicity, was 2.5 (95% confidence interval 1.9-3.3). This odds ratio varied by neither ethnicity nor age of the participants. Sera from 1,087 controls were used to examine the relations between family history and serum concentrations of androgens and prostate-specific antigen. The concentrations of sex hormone-binding globulin were slightly higher in men with than without a positive family history. Prostate-specific antigen concentrations were unrelated to family history.