In the last several years, the development of an effective breast cancer screening procedure has increased the possibility of the early detection of this cancer. We investigated the cost-effectiveness of a breast cancer screening program to screen 100,000 women 50–64 years of age in Catalonia (Spain). The cost-effectiveness ratio was measured in terms of the cost per cancer detected comparing program costs to the estimated number of cancers detected. We assumed a participation rate, sensitivity, and specificity of 70%, 92%, and 94%, respectively, and that 0.36 breast cancers could be detected per 100 women screened. We estimated a total cost of $2.1 million with $1.4 million for the mammographic stage and $0.7 million for the detection of the true positive mammographic results. The cost per woman screened was $30. We estimated that 252 cases of breast cancer could be detected with the program. The cost-effectiveness ratio obtained in this study was $8,424 per cancer detected. Sensitivity analysis has shown that cost-effectiveness results are sensitive to the variations in the following variables: specificity, cancer detection rate, and screening costs. In planning preventive programs, breast cancer screening should be considered one of the priorities.