Trastuzumab (Herceptin) is a monoclonal antibody approved for the treatment of breast cancer that overexpresses human epidermal growth factor receptor 2 (HER2). Well designed clinical trials in women with early breast cancer have demonstrated that 1 years' therapy with adjuvant intravenous trastuzumab (a loading dose followed by 6 mg/kg every 3 weeks or 2 mg/kg weekly) significantly improves disease-free survival and overall survival compared with observation (subsequent to chemotherapy) or chemotherapy alone in women with HER2-positive disease. In the HERA trial, disease-free survival was estimated to improve by 6.3% at 3 years in the trastuzumab group compared with the observation group. Trastuzumab is generally well tolerated. The most common adverse events are infusion-related symptoms, such as fever and chills, which usually occur with administration of the first dose. Cardiotoxicity occurs in a small proportion of patients receiving trastuzumab, particularly when coadministered with anthracyclines, and cardiac assessment is recommended for all patients at baseline and at 3-monthly intervals. In modelled cost-effectiveness analyses based on data from clinical trials in patients with HER2-positive early breast cancer, adjuvant trastuzumab was predicted to be cost effective from a healthcare payer or societal perspective in several countries. Incremental costs per QALY or life-year gained with trastuzumab administered subsequent to or concurrent with chemotherapy compared with chemotherapy alone were consistently within accepted local thresholds for cost effectiveness. Sensitivity analyses demonstrated that these results remained generally robust to plausible changes in key model assumptions. In conclusion, in patients with HER2-positive early breast cancer, the addition of adjuvant trastuzumab is clinically effective in improving disease-free survival. Available pharmacoeconomic data from several countries, despite some inherent limitations, support the use of adjuvant trastuzumab for 1 year as a cost-effective treatment relative to chemotherapy alone in this patient population.