Recent emphasis has focused on the development of an immunotherapeutic approach toward the treatment of breast cancer. In particular, evaluation of antibodies and vaccines are active areas of research. The monoclonal antibody trastuzumab (H), directed against the HER-2/neu protein, has resulted in inhibition of tumor growth in both preclinical and clinical studies. This effect can be increased when used in combination with several chemotherapeutic agents. A randomized trial of chemotherapy alone versus chemotherapy plus H in untreated metastatic breast cancer patients found prolonged survival in the combination therapy arm. Cardiac toxicity was increased with doxorubicin and cyclophosphamide plus H but not for paclitaxel (T) plus H. Several trials of dose-dense weekly T have found minimal toxicity and significant clinical benefit. These findings prompted the initiation of a trial to evaluate weekly 1-h T plus weekly H. Preliminary data from this ongoing study demonstrate few side effects and a response rate of 64% (95%CI 42-76%). The optimal role of H in the treatment of breast cancer has not yet been defined. Additional evaluation in the metastatic and adjuvant settings is planned.