A randomised controlled trial of mass screening for breast cancer by single-view mammography was begun in Sweden in 1977. All women aged 40 and older and resident in the counties of Kopparberg and Ostergötland were enrolled. The present report is confined to the Ostergötland study, which started in 1978 and comprised 92 934 women. After randomisation, which was done on the basis of communities rather than individuals, 47 001 women were allocated to the study group and offered repeated mammographic screening; 45 933 were allocated to the control group. As compliance among women over 74 years of age was poor these were excluded from the present report. The yearly incidence of stage II or more advanced breast cancers after the initial screening round up to and including the second was reduced by 40 per cent in the study group compared with the controls. This effect was less marked in the age group 40-49. After 5.5 years average from the date of entry the absolute number of women with stage II-IV disease in the control group exceeded that for the study group by 44, whereas there was a large excess of cancer in situ and stage I cancer in the study group.