The different rates of breast cancer found between Chinese women in Asia compared with Chinese-born women in the United States suggest that dietary and environmental factors may be of etiological significance. We evaluated the proportion of 480 premenopausal Chinese women who yielded nipple aspirate fluid (NAF) by birthplace in Asia versus the United States and by reproductive and other risk factors. Birthplace was used as a surrogate for presumed differences in exposures during gestation, childhood, and adolescence that might influence yield of NAF in premenopausal women. In United States-born Chinese women compared with Asia-born Chinese women, the proportion yielding NAF was 44 of 95 (46.3%) versus 120 of 385 (31.2%), respectively. The relative risk of yield of NAF in United States-born women compared with Asia-born women was odds ratio = 2.37 (95% confidence interval, 1.26-4.47). Independent positive associations of NAF yield were also found with history of parity and breast feeding, cerumen phenotype, and a negative association with ever use of oral contraceptives. These findings support the hypothesis that early environmental exposures may have long-lasting physiological effects discernible in the breast glands of adult women.