Amplification of c-erbB-2 was at least three times more frequent in breast cancer than in most other types of carcinoma, and was not found in sarcomas or hematological malignancies. Amplification of c-erbB-2 was found in 15 of 86 primary breast cancers and in 3 of 12 secondary breast cancers. Amplification was more common in breast tumors of advanced stage, and in tumors which had metastasized to regional lymph nodes sites. Gene amplification was observed in 21% (4 of 19) of primary tumors which recurred within 3 years of mastectomy and in 6% (2 of 32) of nonrecurrent tumors.