OBJECTIVES: Trastuzumab is a humanized monoclonal antibody against the extracellular domain of HER-2 and it has activity in early and advanced breast cancer with HER-2 overexpression. The cost-effectiveness of trastuzumab in 1 year therapy was assessed for patients with breast cancer who had completed loco-regional surgery and at least four cycles of neo-adjuvant or adjuvant chemotherapy in comparison with observation from the public health care system perspective. METHODS: This cost-effectiveness analysis was based on the HERA trial. A modified Delphi panel with local specialists was conducted to identify local resources usage for treating breast cancer. Costing was based on public sources. A 5-state Markov model was developed to simulate the disease progression: disease-free survival, recurrence, metastatic, cardiac events and death. Only direct costs were considered in the calculation and a lifetime perspective was assumed. A discounting rate of 5% was adopted according to DECIT local guidelines for economic evaluation. Probabilistic sensitivity analysis was performed to account the robustness of the estimates. RESULTS: Trastuzumab treatment costs were higher than those of the observational arm: a R$ 43,363 increment for trastuzumab. The use of trastuzumab reduced time in the metastatic state by 1.15 years and then cost-offsets of R$67,472 were observed as a consequence. For the total period, trastuzumab arm presented an increase in discounted overall survival of 1.36 life years and a discounted quality-adjusted survival of 1.44 QALYs, and also a favorable ICER of R$30,040 per QALY. CONCLUSIONS: This cost-effectiveness analysis suggests that the use of trastuzumab in adjuvant therapy for patients with early breast cancer HER-2 positive brings important clinical benefits to the patients and in addition is a cost-effective alternative within the Brazilian public health care system perspective.