OBJECTIVES: In breast cancer, adjuvant chemotherapy is often prescribed as a precautionary measure and sometimes unnecessarily. A diagnostic test based on an analysis of 21 genes estimating the risk of recurrence at 10 years for women with breast cancer in early stage has been developed (Oncotype-DX®). A cost-effectiveness model was implemented to evaluate the long-term costs and clinical outcomes associated with introducing this test to inform decision on adjuvant chemotherapy in France. METHODS: A Markov model was implemented to evaluate the impact of the test in terms of costs, life-years gained and quality-adjusted life-years (QALYs) in French clinical practice for patients with ER +, HER2-, node-negative early-stage breast cancer, over 30 years. A strategy using the genetic test to decide whether to administer chemotherapy or not was compared to utilization of standard decision criteria. The model accounted for the costs of testing, adjuvant chemotherapy and recurrence. Utilities were dependent on recurrence status and undergoing chemotherapy. Input data were obtained from a study evaluating the proportions of patients in which the genetic test led to change the oncologist’s decision, as well as scientific literature and grey literature. RESULTS: The test was associated with savings of €570 (€1600 with productivity loss cost) per patient from societal perspective and gains of 0.15 life-years and 0.14 QALYs per patient.  One-way sensitivity analyses showed that the cost was most sensitive to the recurrence cost and QALYs to discount rate and to the proportion of patients for whom the decision not to give chemotherapy was reversed with the test. CONCLUSIONS: The use of the test seems to represent efficient use of health care resources in French practice. This test provides an opportunity to optimize treatment prescription by avoiding unnecessary chemotherapies and by prescribing chemotherapy to women who have not received it based on standard decision criteria.