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The prevalence (lifetime occurrence) rate of cancers of the reproductive system (uterus, ovary, cervix, and vagina) and breast was determined for 5,398 living college alumnae, 2,622 of whom were former college athletes and 2,776 nonathletes, from data on medical and reproductive history, athletic training, and diet. The former athletes had a significantly lower risk of cancer of the breast and reproductive system than did the nonathletes. The relative risk (RR), nonathletes/athletes, for cancers of the reproductive system was 2.53, 95% confidence limits (CL) (1.17, 5.47). The RR for breast cancer was 1.86, 95% CL (1.00, 3.47). The analysis controlled for potential confounding factors, including age, family history of cancer, age at menarche, number of pregnancies, use of oral contraceptives, use of estrogen in the menopausal period, smoking, and leanness. Of the college athletes, 82.4% had been on precollege teams compared with 24.9% of the college nonathletes. We conclude that long-term athletic training lowers the risk of breast cancer and cancers of the reproductive system. The lowered risk may be related to changes in estrogen metabolism associated with increased leanness.