€OBJECTIVES: The purpose of this study is to estimate the cost-effectiveness (CE) of 1-year trastuzumab treatment versus standard care (observation following standard adjuvant chemotherapy) in early stage breast cancer (eBC) patients in Portugal. METHODS: A 5-state Markov model with annual transition cycles was developed to estimate the long term health and economic outcomes of eBC patients based on HERA clinical trial results. The model included the following health states: disease free survival, recurrence, metastasis, cardiac events and death. The model assumes a hypothetical patient cohort similar to those of HERA study. The evaluation assumes both the health care payer and societal perspectives. Portuguese NHS resource use and costs were estimated from a consensus experts panel and published unit costs, respectively, including cancer therapy costs, adverse cardiovascular events treatment costs, disease diagnosis and management costs and indirect costs (time off of work). Outcomes were discounted at 3% per annum. One-way sensitivity analysis was performed on the discount rate, quality of life estimates and non-trastuzumab treatment costs. RESULTS: Treatment with trastuzumab was estimated to increase discounted life expectancy by 2.11 in years (14.95 vs 12,84) and quality-adjusted life expectancy by 2.01 QALYs compared to standard care. Direct and indirect costs were projected to be €61.839 and €19,759 with trastuzumab and €40,559 and €25.391 with standard of care. These results corresponded to ICERs of €10,067 and €10,595 assuming direct costs only and of €7789 and €7400 including indirect costs, per life year gained (LYG) and per QALY gained, respectively. CONCLUSIONS: The 1-year trastuzumab use as adjuvant therapy in HER-2 positive eBC patients improves survival and can be considered a cost effective therapy with a high degree of certainty in the Portuguese setting