Germline mutations in the BRCA1 gene predispose to breast and ovarian cancer. An estimated 45% of families with multiple breast cancer cases and more than 80% of breast-ovarian cancer families are linked to BRCA1. Mutation analyses by collaborative laboratories have revealed around 460 distinct BRCA1 sequence alterations, mostly germline mutations from familial cases. The majority of these alterations were nonsense and frame-shift mutations. In the present study, breast tumors of both sporadic and familial origin were investigated for allelic imbalance (AI) at the BRCA1 locus. AI was observed in 52% of the sporadic cases and in 17% of the familial cases. Furthermore, 104 breast carcinomas from patients with sporadic disease and 77 patients with positive family histories of breast and/or ovarian cancer were examined for translation-terminating mutations in exon 11 of the BRCA1 gene using the protein truncation test (PTT). No somatic mutations were detected in any of the tumors analysed, and only one BRCA1 mutation carrier was found among the familial cases. The result of this study gives no indication that truncating somatic mutations in exon 11 of BRCA1 play a major role in the tumorigenesis of the breast. Furthermore, the frequency of such mutation carriers in breast cancer populations with weak family histories of breast and/or ovarian cancer seems to be low.