Trastuzumab (Herceptin), a recombinant, humanised, monoclonal-antibody that targets human epidermal growth factor receptor 2 (HER2), has been approved as an adjuvant therapy for HER2-positive early breast cancer. The aim of this study was to assess the incremental cost-effectiveness ratio of this treatment compared with adjuvant therapy alone in the French setting. A cost-effectiveness analysis was performed using a Markov state transition model. The transition probabilities were estimated from the interim results of the Hera trial. Unit costs data were mainly estimated in a French Oncology Center (Georges-François Leclerc, Dijon). The model estimated that overall mean survival of patients treated with trastuzumab was 20.08 years versus 16.23 in the observation group (3.85 life-years gained). For 1 000 patients with a 10-year follow-up, an adjuvant therapy with trastuzumab would avoid 49.7 loco-regional recurrences, 179.5 distant recurrences and 133.4 deaths. The incremental discounted cost of trastuzumab therapy over a lifetime horizon was estimated at 27594 euro per patient in association with a discounted gain of 2.27 life-years. In accordance with the techniques of economical evaluation, the utilization of trastuzumab as an adjuvant therapy in patients with early HER2 positive breast cancer improves patient survival with an acceptable cost-effectiveness ratio in the French setting (incremental cost-effectiveness ratio of 12,148 euros /LYG).