Objective To assess the impact of the national breast cancer screening programme on breast cancer mortality in the first years after its introduction. Setting The Netherlands and United Kingdom. Methods —MISCAN models, incorporating demographic, epidemiological, and screening characteristics of the region under study, were used to assess the mortality in the presence and absence of screening. Results Breast cancer mortality decreased in women aged 55–74 as the Dutch nationwide screening programme built up, and was 5% lower in 1996 than before the start of the programme. The mortality reduction due to screening in the age group 55–74 is expected to increase gradually to 18% in 1999, 10 years after the introduction of screening, and to 29% in the long term. In the United Kingdom screening was expected to achieve a mortality reduction of 5% and 18% in the age group 55–69 five and 10 years respectively after screening was started. A maximum mortality reduction of 24% in this age group is predicted. Conclusions —The effects of screening will be small in the first years after the start of the programme. Accordingly, it was expected that the reduction in breast cancer mortality due to the Dutch nationwide breast screening programme, which started around 1989, would be statistically significant from 1997 onwards, the point at which the target population of women was completely covered; 70% of the reported 12% mortality reduction in England and Wales in 1994 is expected to be attributed to screening.