OBJECTIVES: In the Basque Country (Spain), mammographies have been done in biennial basis to women in their fifties and sixties since 1996. The main objective of this project was the evaluation of the impact of the Screening program in terms of costs and health in the Basque women population since 1996. METHODS: A discrete event simulation model was built to represent the natural history of breast cancer in women invited to the breast cancer screening program in the Basque Country. The disease progress was described in three main states (healthy, preclinical and clinical) in the model. We assumed all women would be diagnosed at the beginning of the clinical stage unless it had been diagnosed previously through the screening program. The data collected among the 15 years when the screening program was held allowed model’s validation. In order to compare the economic impact of these scenarios mammography and treatment costs – depending on the diseasestage at diagnosis – were included. The health impact assessment was based on quality adjusted life expectancy of cancer patients. RESULTS: Since the screening program started working, 8,925 cancers were detected among 313,475 women who attended the screening which represents the 76% of the invited ones. 60% of the diagnosed cancers were detected through the screening program. All the mammographies carried out during the evaluated years costed 46 million Euros. Each cancer detected in the screened scenario costs 29,581.06€, in the background scenario the cost was 29,639.22€. In terms of total costs the background scenario had a lower cost on average until the year 2009. CONCLUSIONS: Early detection of breast cancer improves survival prognosis and decreases treatment costs for each detected cancer. In the future, the costs of the early detection program will be balanced by the savings in treatment costs.