BRCA1 protein is normally expressed in the nuclei of breast epithelial cells and functions as a negative regulator of the cell cycle. Mutations of BRCA1 gene have been associated with familial breast carcinomas. Although somatic mutations have not been demonstrated in sporadic breast carcinomas, loss of nuclear expression of BRCA1 protein has been correlated with high-grade, infiltrating breast carcinomas in females. We have investigated the expression of BRCA1 protein in a series of 26 infiltrating carcinomas of male breast and correlated the results with a variety of clinicopathological parameters. Immunohistochemical analysis demonstrated decreased expression of BRCA1 in all carcinomas compared to normal breast epithelium. The median survival was 8 years in patients with minimal loss of BRCA1 expression while it was 1.5 years in patients with marked loss of this protein. Marked loss of BRCA1 protein was also associated with increased cell proliferation. These results suggest that BRCA1 plays a similar role in both male and female breast carcinoma and Joss of this protein is associated with poor prognosis.