OBJECTIVES: In 2007 trastuzumab (T) received regulatory approval in Brazil for 1 year treatment of HER2-positive patients with early breast cancer (eBC) in addition to standard chemotherapy. The objective of this analysis is to estimate cost-effectiveness of T in eBC from the private third-party payer perspective. METHODS: A Markov state transition model was designed to capture the natural history and course of disease for early stage breast cancer patients, and to simulate cost and disease progression over a life time perspective. Disease transition probabilities and clinical outcomes were derived from the HERA trial. Direct costs were considered to estimate the incremental cost-effectiveness ratio using both quality-adjusted life years (QALYs) and life years gained (LYGs). A local Delphi panel was conducted to reflect local clinical practice and assess medical care utilization (for disease-free survival, loco-regional recurrence, metastasis, and cardiac side effects). Unit costs were based on published sources (drugs and materials national prices list and medical society fees list). Utilities and disutilities were obtained from published literature. The discount rate of 3% was adopted for both health outcomes and costs. Both one-way and probabilistic sensitivity analysis were performed to verify the robustness of the results. RESULTS: T treatment costs were higher in comparison with standard treatment, showing higher acquisition drug costs (R$ 190,554) and administration costs (R$ 2,160). On the other hand, cost-offsets for R$ 168,331 were estimated due to the reduction of the metastatic and recurrence cases. T treatment showed an increase in LYG (1.74) and QALYs (1.80). The estimated ICER was R$ 22,587 per QALY gained. CONCLUSION: From a third-party payer, this analysis suggests that trastuzumab treatment for HER-2 positive patients with early breast cancer is a cost-effective alternative in Brazil.