The purpose of this investigation is to study the frequency and penetrance of BRCA1 and BRCA2 germline mutations in Japanese familial breast cancer patients. Mutation analysis of BRCA1 and BRCA2 by protein truncation test was conducted on the 120 breast cancer patients (probands) with at least one breast cancer (site-specific breast cancer families, n=105) or one ovarian cancer (breast/ovarian cancer families, n=15) patient in their first-degree relatives. Eight BRCA1 (7.6%) and ten BRCA2 (9.5%) mutations were found in site-specific breast cancer families (n=105), and seven BRCA1 (46.7%) but no BRCA2 (0%) mutations were found in breast/ovarian cancer families (n=15). In site-specific breast cancer families, mutation frequency of BRCA1 and BRCA2 was high in families with more than three breast cancer patients (30%, 6/20), early onset (40≤ years old) breast cancer patients (41.1%, 14/34), or bilateral breast cancer patients (40%, 6/15). Cumulative incidence of breast cancer by age 70 was estimated to be 78% and 80% for BRCA1 and BRCA2 mutation carriers, respectively, and that of ovarian cancer was 40% and 0% for BRCA1 and BRCA2 mutation carriers, respectively. Family profiles are important determinants of risk for carrying a BRCA1 or BRCA2 mutation. Japanese women with BRCA1 mutation have a high risk of breast and ovarian cancer and those with BRCA2 mutation have a high risk of breast cancer but not ovarian cancer.