The invention provides a method for more effective treatment of patients susceptible to or diagnosed with tumors overexpressing HER2, as determined by a gene amplification assay, with a HER2 antibody. Such method comprises administering a cancer-treating dose of the HER2 antibody, preferably in addition to chemotherapeutic agents, to a subject in whose tumor cells her2 has been found to be amplified e.g., by fluorescent in situ hybridization.