Results from several randomised mammography screening trials have shown that it is possible to reduce mortality in breast cancer by mammographic screening at least for women above 50 years of age. The purpose of this article is to present data on mortality in breast cancer in study and control groups of the Stockholm trial after 11 years of followup, to analyse which age group benefits most from screening. In March 1981, 40,318 women in Stockholm, aged 40 through 64 years, entered a randomized trial of breast cancer screening by single view mammography alone, versus no intervention in a control group of 20,000 women. Two screening rounds were performed and the attendance rate was over 80% in the two rounds. During 1986 the control group was invited once to screening. Totally 428 and 217 cases of breast cancer were diagnosed in the study and control groups respectively. After a mean follow-up of 11.4 years a nonsignificant mortality reduction of 26% was observed for the whole study group, with a relative risk (RR) of death in breast cancer of 0.74 (CI(confidence interval) = 0.5-1.1). For women aged 50-64 years a significant 38% mortality reduction was observed with a RR of 0.62 (CI = 0.38-1.0). For women aged 40-49 years no effect on mortality was found, with a RR of death in breast cancer of 1.08 (CI = 0.54-2.17). The breakpoint for benefit in this study seemed to be at 50 years of age when 5-year age groups were analysed, but this tendency is uncertain because of the low statistical power in the analysis of the younger age groups. Long screening intervals, the use of single-view mammography, and the fact that more than 50% of the women in age group 40-49 years were still below 50 years of age when the study was closed, were all facts that could have influenced the results in age group 40-49 years. Larger studies are needed to answer the question whether mammographic screening can be successful in younger age groups.