A randomized trial is presented on the effect of repeated invitation to breast cancer screening with mammography on mortality from breast cancer. The invited group and the control group each consisted of approximately 21,000 women aged 45-69 yr at the start of the screening. The attendance rate was 74% at the first screening and 70% at the two subsequent screening rounds. The cancer detection rate was 7.5 per 1000 women examined in the first screening round and 2.2 and 2.0 per 1000 woman-years in the second and third screenings with an incidence of 0.9 in the intervals. The incidence in the control group was 2.7 per 1000 woman-years. The proportion of positive biopsies was 61% in the first screening round, 33% in the second, and 58% in the third. After the prevalence screening, the stage distribution was more favourable in the invited group (including non-attenders) than in the control group. In the two most recent periods of the programme, 62 out of 160 women with cancer (39%) in the invited group were in stage II-IV compared with 91 out of 159 (57%) in the control group. More than 60% of cancers detected at screening were either non-invasive or invasive with a diameter of 1 cm. The corresponding percentage in the control group was 27%. The importance of sampling bias is discussed. Although data on mortality still have to be awaited, the results so far clearly indicate a positive effect of screening.