HER-2 (c-erbB-2, neu) is an important prognostic and predictive factor in breast cancer. Clinical trials utilizing a humanized version of the anti-HER-2 murine monoclonal antibody 4DS, trastuzumab (Herceptin; Genentech Inc, South San Francisco, CA), have shown antitumor activity in patients with HER-2-positive metastatic breast cancer. Improved response and survival rates have been shown when trastuzumab was added to first-line combination chemotherapy with anthracycline/cyclophosphamide or paclitaxel, compared to the same chemotherapy alone. The Breast Cancer Intergroup has recently completed several trials evaluating new chemotherapy treatment approaches for patients with node-positive breast cancer, which form the basis for several ongoing and planned clinical trials incorporating trastuzumab. These clinical trials and the evolving role of trastuzumab-containing adjuvant systemic therapy for breast cancer will be reviewed.