In order to examine the role of the erbB-2 oncogene in human breast cancer, gene amplification and expression were examined in multiple stages of tumor progression. Gene amplification ranging from 2-fold to 32-fold was found in 30 (29%) of 130 cases analyzed. Expression of the receptor-like gene product was determined by a combination of Western immunoblotting and immunohistochemistry. In each case of gene amplification, there was high level overexpression (+ + +) of the protein product. In an additional 29 of 111 cases in which expression was studied (26%), there was moderate level overexpression (+ +) of erbB-2 in the absence of gene amplification. Amplification and overexpression of the erbB-2 gene were found in early clinical stages of breast cancer as well as in more advanced cases. In 23 patients, gene number and level of gene expression were equivalent in the primary tumor site compared with single or multiple metastatic sites in regional lymph nodes. Using a combination of immunohistochemistry and in situ cytohybridization, high (+ + +) and moderate (+ +) level overexpression were homogeneously present in all malignant epithelial cells within histological sections of both primary and metastatic tumor. The intraductal component of carcinoma was identified in sections from 16 invasive primary tumors. erbB-2 gene expression in the intraductal lesions was equivalent to or exceeded expression in the infiltrating components of these tumors. Because erbB-2 alterations are (a) present in all clinical stages, (b) maintained during metastatic spread, (c) homogeneously present throughout tumor sections, and (d) present in the in situ as well as infiltrating component, we conclude that these alterations are selected for early and may be important in the initiation of certain mammary cancer.