A mathematical model of breast cancer is developed and used to evaluate the benefits of screening for breast cancer. This model consists of hypotheses concerning the age-specific incidence of the disease, the rate of disease progression, the tendency of the disease to be detected without benefit of scheduled screening examinations, and prognosis related to the extent of disease progression at treatment. We formulate these hypotheses quantitatively and estimate parameters by fitting the model statistically to published data on breast cancer. Model predictions are independently validated by comparison with data from breast cancer screening programs. On the basis of the model, we calculate the benefits of screening under alternative assumptions about the woman screened, the number of screens given, the ages at which the screens are given, the reliability of the screening technique, and the rate of disease progression. These calculations are then used to consider questions concerning the design of breast cancer screening strategies.