The expression of c-erbB-2 oncogene and p53 tumor suppressor gene was studied in methacarn-fixed, paraffin-embedded biopsy specimens from 58 benign breast lesions and 129 sporadic breast carcinomas, using the supersensitive monoclonal antibodies CB 11 and BP 53-12-1 and the biotin-streptAvidin-amplified methodology. None of the benign lesions studied, which included 36 fibrocystic lesions with mild or florid epithelial hyperplasia, 12 fibrocystic lesions with ADH or ALH and 10 fibroadenomas, demonstrated membrane staining for c-erbB-2 or nuclear immunoreactivity for p53. Overall, 48.06% of primary breast carcinomas showed membrane expression of c-erbB-2, while 28.68% were p53 positive. Those showing p53 immunoreactivity displayed a nuclear and/or cytoplasmic staining type. A significant correlation was seen between c-erbB-2 and p53 expression (r = 0.213, p < 0.05), as well as between c-erbB-2 status and PSNA score (r = 0.221, p < 0.05). In addition, c-erbB-2 and p53, separately or in combination, correlated significantly with the prognostic index. In conclusion, immunohistochemistry of c-erbB-2 and p53 immunohistochemistry allows a better definition of intraductal proliferative lesions and assists in the differentiation between ADH and DCIS. It provides additional clues with regard to the biologic behavior of invasive ductal carcinomas (NOS and medullary).