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The issue of whether to screen women aged 40–49 for breast cancer is debated usually in terms of the potential mortality reduction achievable by the application of screening in this age group. Theories regarding why a significant reduction in mortality has not been observed in trials relate to the biologic behavior of tumors in this age group and the screening process itself. Survival curves with respect to node status, size, and grade of the tumor were compared among age groups in the Swedish two‐county trial. In the Kopparberg part of this trial, for the 40–49 age group, predicted survival was calculated from the size, node status, and grade of cancers detected during the trial in comparison with those found in two later series of tumors, one from the 1989–1992 Kopparberg screening program, the other from the British Columbia screening program that began in 1988. The Kopparberg arm of the Swedish two‐county study used single‐view mammography with extended processing but without grid; the two more recent programs used two‐view mammography with extended processing and the grid. Both the Kopparberg programs used a 2‐year interval. The effects of grade, node status, and size on survival in the 40–49 age group were very similar to their effects in older age groups. Predicted survival from the later Kopparberg series was essentially the same as that for the earlier. The mortality reduction in this age group in the Kopparberg part of the Swedish two‐county trial was 26%. The survival results indicate no biologic reason why screening should not be able, theoretically, to reduce mortality. Nonsignificant reductions in mortality have been observed in the Kopparberg part of the two‐county trial and in the overview of Swedish trials. The similar predictive results for the two‐view and one‐view trials suggest that the most likely way to achieve further reductions in mortality is to reduce the interval between screens, possibly to 1 year.

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