Trastuzumab (Herceptin) is a humanized IgG(1) kappa monoclonal antibody, specifically targeted against the extracellular domain of the human epidermal growth factor receptor 2 (HER2), and is indicated for the treatment of HER2-positive early or metastatic breast cancer. This review discusses the available data regarding its use in early breast cancer. Trastuzumab, when administered concurrently with chemotherapy regimens, consistently prolonged disease-free survival (primary endpoint) and overall survival (secondary endpoint) in patients with HER2-positive early breast cancer in well designed trials; studies evaluating sequential trastuzumab treatment have produced mixed results for these endpoints. Further study is required to ascertain the optimal trastuzumab treatment regimen, including the duration of treatment. Trastuzumab was generally well tolerated when added to, or administered following, a chemotherapy regimen in clinical trials. While cardiac adverse events, such as a decreased left ventricular ejection fraction and congestive heart failure, are a concern, these effects are treatable and appear to be mostly reversible. Thus, trastuzumab is a valuable component of treatment regimens for HER2-positive early breast cancer.