Estrone ( E l ) , mtradiol ( E 2 ) and m r r i o l ( E 3 ) were measured in the urine of Asian and North American women aged 15-19, 20-24 and 30-39 A woman's breast-cancer risk may be related to the pattern of her estrogen metabolism during young adulthood. Specifically, it has been hypothesized that women who, in the decade or so after puberty, have relatively low ratios of cestriol (E3) to estrone ( E l ) and estradiol (E2) are at high risk of breast-cancer (Cole and MacMahon, 1969). The long interval between puberty and the usual age at clinical appearance of breast cancer make this hypothesis difficult to evaluate directly. One method of indirect assessment is to determine whether estrogen profiles of young women vary between populations in ways that would be predicted from knowledge of the populations' breast-cancer rates. A pilot study, comparing urine estrogen profiles of Asian and North American women, provided support for the hypothesis (MacMahon et al., 1971). The larger study which ensued is described herein.
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