For some years, there has been a perceived need for more information on the effect of screening for breast cancer in women aged 40 to 49. Our approach was to gather the most recent data on screening in this age group, to assess the following quantities: the likely benefit in mortality terms, measures of screening performance and arrest of tumour progression through screening, costs and public-health implications, and prospects for future screening and research. A collaborative meeting was held in Falun, Sweden, for which data were gathered in advance from all the randomized trials of breast-cancer screening that included women in this age group, and all identifiable substantial databases on service screening of women aged 40 to 49. Updated results from the Swedish overview of mammographic screening trials indicated relative mortality associated with invitation to screening of 0.77 (95% confidence interval 0.59-1.01). Combining all population-based randomized trials gave the relative-mortality figure of 0.76 (0.62-0.93), and combining all trials gave 0.85 (0.71-1.01). Relative-mortality figures in individual trials could be predicted by the rates of advanced cancers in those trials. Detailed analysis suggested faster tumour progression in the age group 40 to 49 compared with groups aged 50 or more. It is likely that mammographic screening of women aged 40 to 49 can reduce subsequent mortality from breast cancer, and studies on tumour progression indicate that to obtain substantial benefit it is probably necessary to screen every 12 to 18 months, with 2-view mammography and double reading of films. This represents a greater outlay in resources and in commitment of the population. Further work remains to be done on the time taken for a mortality benefit to emerge, on age at diagnosis in some of the trials, and on costs and public-health aspects.