p53 mutation is a common event in sporadic breast cancer being found in 15-50% of invasive carcinomas. The purpose of this study was to determine the earliest histologic stage at which p53 mutation could be detected with a widely used anti-p53 antibody (DO7, Novocastra) which recognizes both wild type and mutant forms. p53 expression was assessed immunohistochemically in 12 primary breast carcinomas with known p53 mutations and in all pre-malignant epithelial lesions surrounding these invasive cancers. Strong p53 nuclear staining was found in all of the tumors known to have missense mutations and none of the tumors with truncation mutations. In cases with intense staining in the invasive carcinoma, a similar quality of staining was also seen in all areas of DCIS (ductal carcinoma in situ) and was representative of missense p53 mutations. Lighter nuclear staining intensity was observed in up to 40% of cells in areas of hyperplasia and in up to 30% of normal breast lobules irrespective of the type of mutation found in the invasive carcinoma. This weak staining was not specific to mutated p53 and may indicate increased amounts of normal p53 protein. We conclude that p53 inactivation occurs prior to invasion in breast carcinogenesis, with mutations being uniformly identified in DCIS associated with p53-mutated invasive carcinomas. In contrast, there is no evidence that epithelial hyperplasia or epithelial cells of the terminal duct lobular unit harbor the same mutations as their associated invasive carcinoma.