DISEASE CHARACTERISTICS: HER2 overexpressing (3+) metastatic breast cancer by immunohistochemistry, or amplified by fluorescent in situ hybridization (FISH) No prior chemotherapy for breast cancer metastases No bilateral disease Bidimensionally measurable disease At least 1 target lesion greater than 1 cm2 No significant lymphedema in the arm ipsilateral to mastectomy site, unless patient has an indwelling catheter for the purpose of chemotherapy infusion No CNS metastases Hormone receptor status: Not specified