BACKGROUND: Recent case-control studies on the effectiveness of population-based breast cancer screening show differences in the magnitude of breast cancer mortality reduction. We investigated the role played by aspects of the case-control study design on these differences, e.g. the definition of cases and exposure to screening. Material and methods: We investigated six case-control studies conducted in East Anglia (UK), Wales, Iceland, central and northern Italy, South Australia and The Netherlands. RESULTS: The breast cancer mortality reduction in the different case-control studies ranged from 38% to 70% in the screened versus the nonscreened women. We identified differences in design, e.g. the inclusion or exclusion of the first years of screening, and the correction factor for self-selection bias. CONCLUSIONS: Overall, the design of the case-control studies was similar. The differences in the magnitude of breast cancer mortality reductions are therefore unlikely to be caused by variations in the design of the case-control studies. These differences must be due to other factors, like the organisation of the service screening programme and the attendance rate. The reduction in breast cancer mortality estimated in these case-control studies indicates that the impact of current mammographic screening is at least consistent with the effect reported by the former randomised screening trials.