Amplification of the her-2/neu gene is associated with poor prognosis in patients with early-stage and metastatic breast cancer. Trastuzumab is a humanized monoclonal antibody directed against the HER2 protein, which is overexpressed in approximately 25% of patients with primary invasive breast cancer. Randomized phase III clinical trials showed that administration of trastuzumab in combination with chemotherapy or following chemotherapy significantly improves disease-free and overall survival rates in patients with early-stage HER2-positive breast cancer. The integration of trastuzumab in the adjuvant setting is changing the natural history of HER2-driven breast cancer from one of the worst subtypes to one that is highly curable with available therapy.