The costs and effects of different invitation schedules of breast cancer screening are compared. The effect estimates are based on trials from the USA, Sweden and the Netherlands. The cost estimates use registration data, file studies and organization charts. The calculations were performed with the MISCAN computer simulation package, which is developed especially for the evaluation of mass screening programmes. Screening women of 50-70 years at 2-yearly intervals is a relatively cost-effective schedule. In a real population, it will reduce breast cancer mortality by 12%. Screening of women under 50 is probably far less cost-effective. Screening induces a considerable shift towards breast-conserving therapy. Although a 12% mortality reduction may seem low, in absolute numbers this represents more than the total mortality from, e.g., cervical cancer. Moreover, cost per death prevented or per life-year saved is much lower than for most other medical interventions for which cost-effectiveness ratios are known, screening for cervical cancer included.