Overexpression of the nuclear phosphoprotein p53 is one of the most frequently detected abnormalities in human cancer and appears to be associated with mutation of the p53 gene. In this study of breast cancer, p53 overexpression was detected in two (15%) of 15 pure intraductal tumors, 73 (25%) of 291 primary invasive carcinomas, 13 (50%) of 26 lymph nodes containing metastatic breast cancer, and two of four established breast cancer cell lines. Sequence analysis of selected specimens confirmed that p53 overexpression was associated with mutation of the gene, while no mutations were detected in specimens without p53 overexpression. Thus, overexpression of p53 occurs in all stages of breast cancer and is consistently associated with the production of mutant proteins. Immunohistochemical analysis is a simple method which reliably predicts the presence of most p53 gene mutations in breast cancer specimens.