Clinical trials using the humanized version of the anti-HER2 murine monoclonal antibody 4D5, trastuzumab, have shown antitumor activity in patients with HER2-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 with the same chemotherapy alone. Because of the promising safety and efficacy profile of trastuzumab in the metastatic setting, this novel biologic has now entered adjuvant breast cancer trials in both the United States and Europe. Five clinical trials are discussed in this report, capturing the evolving role of trastuzumab as adjuvant therapy for breast cancer.