In March 1981 a randomized single-view mammographic screening for breast cancer was started in the south of Stockholm. The screened population in the first round numbered 40,318 women, and 20,000 women served as a well-defined control group. The age groups represented were 40-64 years, and 80.7% of the invited women participated in the study. The first round disclosed 128 breast cancers (113 invasive and 15 noninvasive), or 4.0 per 1,000 women. Mean tumour size was 14.1 mm and axillary lymph node metastases were found in 21.8%. Fifty-five per cent of the tumours were small (less than or equal to 10 mm) or non-invasive, and 71% were stage I. Participation rates are high in all Swedish trials. The present results differ only slightly from other screening programs; the percentages of patients with axillary metastases and stage II tumours are similar in the Stockholm, Malmö and Kopparberg/Ostergötland studies. Comparisons of cancer prevalence in the various Swedish screening trials show that, in comparable age groups, there are some differences, even when the differences in the natural cancer incidence are taken into account. A decreased mortality was found recently in a Swedish trial in ages above 50 years but not below. In the Stockholm study more than one-third of the participants were aged 40-49 years.