Fifteen (13.3%) and 21 (18.6%) deleterious germline mutations were identified in BRCA1, and BRCA2 genes among 113 Japanese breast cancer families. We found a BRCA1 codon 63 (nucleotide 307) nonsense mutation and a 4-bp deletion at codon 1858 (nucleotide 5802) of BRCA2 in 4 and 7 independent families, respectively. Haplotype analysis has revealed that these two mutations were founder mutations. Lifetime risk of breast cancer in BRCA1 or BRCA2 mutation carriers was estimated at nearly 80%, and that of ovarian cancer in BRCA1 mutation carriers was about 40%. A questionnaire survey as to the genetic testing (BRCA1 and BRCA2) and prevention was carried out with 146 healthy women (hospital workers or medical students) and 84 breast cancer patients. About 80% of healthy women as well as breast cancer patients responded positively to the genetic testing, based on the assumption their's was a breast cancer family, and about 20% of each group answered that they would opt for prophylactic mastectomy and oophorectomy if they were found to be germline mutation carriers. These results indicate that genetic testing and prophylactic surgery would be acceptable among a considerable number of Japanese women, and seem to support the establishment an infrastructure for genetic testing in Japan.