One hundred and thirty-seven female breast cancer patients have been HLA-typed by a cytotoxicity test. Over all, no specific HLA antigen was found, though when the patients were divided into two groups, i.e., into those with bilateral or unilateral cancers, A24 and Cw7 was found to be significantly increased in the bilateral group. Further, a haplotype of A24-Cw7 was frequently seen in the bilateral group. No specific HLA antigen, however, was found in patients stratified by a familial history of cancer. It thus was concluded that bilateral breast cancer patients that have developed from patients with a unilateral cancer are detectable by HLA typing.