Mammographic screening for women aged 50-70 is effective in reducing breast cancer mortality, but the impact on quality of life and the attainable mortality reduction remain to be discussed. The consequences of expanding screening programmes to include women in other age groups are uncertain. We have predicted the effects and costs for 5 popular screening variants, differing in age group and screening interval, on the basis of our analysis of the Dutch screening trials and of the reported mortality reductions in other trials. We have also investigated the influence of a large number of uncertain factors. Screening for women aged 50 and over with a 2- or 3-year interval is very cost-effective and will result in reductions of respectively 16% or 10% in breast cancer mortality in a real population. Variation of most variables keeps the cost-effectiveness (CE) ratio limited to the range of US $3,000 to 5,000 per life-year gained. A 2- to 3-fold change in CE ratio would only occur if the extreme estimates of mortality reduction in the Swedish screening trials were applied. The impact on quality of life (QoL) is limited: for the 2-yearly screening policy for women aged 50-70, the cost per Quality-Adjusted Life-Year (QALY) gained is 4,050, whereas the cost per life-year gained is US $3,825. The CE ratio for 2-yearly screening of women aged 40-70 is 5,400, but the additional cost per additional life-year gained is US $35,000. It would be preferable by far to extend the screening programme to women over the age of 70 or to shorten the screening interval for women aged 50-70. Screening performances, the demand for mammograms outside screening and the possibility of a survival improvement irrespective of screening have a strong impact on QoL and CE.