BACKGROUND: Little is known regarding the clinical features of hereditary prostate carcinoma (HPC) and whether other malignancies are associated with this disease. The aim of this study was to investigate whether tumors other than prostate carcinoma aggregate in families with HPC or whether this disease can be considered site specific. METHODS: From 62 Swedish families with HPC, a cohort was constructed of 1364 first-degree relatives of the men with prostate carcinoma in these families. Through linkage to the Swedish Cancer Register, all reported cancer between 1958 and 1996 was identified. The expected number of cases was calculated by using the population rates in Sweden. RESULTS: A standardized incidence ratio (SIR) of 1. 16 (95% confidence interval [95% CI], 0.97-1.38) for the overall cancer risk was observed among the 1364 first-degree relatives. However, significant increased risks were noticed for gastric carcinoma (SIR, 2.78; 95% CI, 1.59-4.52), for breast carcinoma in women (SIR, 1.58; 95% CI, 1.01-2.35), and for kidney carcinoma (SIR, 2.51; 95% CI, 1.15-4.77).The excess risk for breast carcinoma was even more pronounced among women before the age of 65 years in families with earlier onset prostate carcinoma (SIR, 3.64; 95% CI, 1. 66-6.91). Seven families with at least two or more relatives with breast, gastric, or kidney carcinoma were identified, and, in one family, four relatives with early onset gastric carcinoma were observed. CONCLUSIONS: In most of the families with HPC, the disease appears to be "site specific," with no excess of other malignancies. However, in a subset of families, a significant aggregation of prostate carcinoma together with breast carcinoma and/or gastric carcinoma was observed that may have been caused by a common germline mutation in a cancer susceptibility gene.