Tumors are identified as responsive to treatment with anti-HER2 antibodies by detecting the presence of a HER2/HER3 and/or HER2/HER1 protein complex or for HER2 phosphorylation in a sample of tumor cells. Patients suffering from a tumor comprising HER/2/HER1 and/or HER2/HER3 heterodimers and/or HER2 phosphorylation are treated with anti-HER2 antibodies, such as rhuMAb 2C4.