Background. Several studies have found a smaller effect of breast cancer screening on breast cancer mortality in women aged younger than 50 years compared with older women. Various possible reasons have been suggested for this, but none firmly is established. Methods. The Swedish Two‐County Study is a randomized trial of breast cancer screening of women aged 40‐74 years, comprising with 133,065 women with a 13‐year follow‐up of 2467 cancers. The Breast Cancer Detection Demonstration Project (BCDDP) is a nonrandomized screening program in the United States, with a 14‐year follow‐up of 3778 cancers in women aged 40‐74 years. The Swedish results by age were updated. The lesser effect of screening at ages 40‐49 years was investigated in terms of sojourn time (the duration of the preclinical but detectable phase) size, lymph node status, and histologic type of the tumors diagnosed in the Swedish Study and their subsequent effect on survival using survival data from both studies. Results. In the Swedish Trial, a 30% reduction in mortality associated with the invitation to screening of women aged 40‐74 years was maintained after 13‐years of follow‐up. The reduction was 34% for women aged 50‐74 years and 13% for women aged 40‐49 years. Results indicated that the reduced effect on mortality for women aged 40‐49 years was due to a differential effect of screening on the prognostic factors of tumor size, lymph node status, and histologic type. The mean sojourn times in the age groups 40‐49 years, 50‐59 years, 60‐69 years, and 70‐74 years were 1.7,3.3,3.8, and 2.6 years, respectively. Conclusions. These results suggest that much, a though not all, of the smaller effect of screening on mo tality in women aged 40‐49 years is due to faster progression of a substantial proportion of tumors in this age group and the rapid increase in incidence during this decade of life. It is concluded that the interval between screenings should be shortened to achieve a greater benefit in this age group. It is estimated that a 19% reduction in mortality would result from an annual screening regime.

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Background. Eight randomized controlled trials (RCTs) of screening mammography were conducted involving women ages 40 to 49 years at entry. Current data gathered for periods ranging from 7 to 18 years of follow‐up are available from these trials. Methods. Meta‐analyses were performed using a Mantel‐Haenszel estimator method to combine current follow‐up data from the eight RCTs of mammography that included women ages 40 to 49 years. Results. Combining all current data on women ages 40 to 49 years at entry into the trials yielded a 14% benefit from screening mammography, without statistical significance at the 95% confidence level. Combining all data on women ages 40 to 49 years at entry, excluding results from the Canadian National Breast Screening Study, yielded a 23% benefit to women invited for screening, with statistical significance at the 95% confidence level. Conclusions. These results suggest that screening mammography in women ages 40 to 49 years at entry can reduce mortality from breast cancer when combined with adequate follow‐up. Cancer 1995;75:1619‐26.

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