Background: Mutations of the tumor suppressor gene p53 have been identified in breast cancer cell lines, and some breast carcinomas are detectable by immunohistochemical assay because of p53 protein accumulation. Purpose: This study was designed to determine whether p53 protein accumulation in breast cancers correlates with p53 gene mutation, with survival, and with five pathobiologic factors associated with prognosis. Methods: IgGl monoclonal antibody to human p53 protein (PAb 1801) and immunohistochemical methods were used to detect p53 protein accumulation in archival formalin-fixed, paraffin-embedded, randomly selected carcinomas. We studied 295 invasive ductal carcinomas from the Massachusetts General Hospital; 151 were determined to be sporadic (not hereditary). We also studied 97 invasive ductal carcinomas—21 sporadic and 76 familial (hereditary)—from Creighton University. In addition, we examined 31 archival in situ carcinomas, 15 snap-frozen invasive ductal carcinomas, primary cell cultures from three benign breast tissue samples, and breast carcinoma cell lines MDA-MB-231 and MDA-MB-468. Results: Nuclear p53 protein was observed in 16% of the 31 in situ carcinomas, 22% of the 172 sporadic carcinomas, 34% of the 50 tumors from patients with familial breast cancer, 52% of the 23 tumors from patients with the familial breast and ovarian cancer syndrome, and all three tumors from two patients with the Li-Fraumeni syndrome. There was complete concordance between p53 gene mutation and p53 protein accumulation in the 15 snap-frozen carcinomas and in both breast carcinoma cell lines. Statistically significant associations of p53 protein accumulation with estrogen receptor negativity and with high nuclear grade were found. There were statistically significant associations, independent of other prognostic factors, between p53 protein accumulation and metastasis-free and overall survival, for randomly accrued and for both sporadic and familial tumors. Conclusions: Immu-nohistochemically detected p53 protein accumulation was an independent marker of shortened survival and was seen more often in familial than in sporadic carcinomas. Our findings also suggest a correlation between p53 protein accumulation was an independent marker of shortened survival and was seen more often in familial than in sporadic carcinomas. Our findings also suggest a correlation between p53 protein accumulation and p53 gene mutation. [J Natl Cancer Inst 84: 845–855, 1992]