Markedly lower breast cancer incidence rates in Asians than Caucasians are not explained by established adult risk factors. Migration studies suggest the importance of early-life exposures, including perhaps the in utero period. Concentrations of steroid hormones and insulin-like growth factors (IGF) were measured in umbilical cord sera from pregnancies in Shanghai, China (n = 121) and Boston, MA (n = 111). Pregnancy characteristics were ascertained by interview and medical records. Means and percent differences in hormone concentrations comparing Chinese with Caucasians and 95% confidence intervals were estimated from linear regression models. Cord concentrations of androstenedione (91.9%), testosterone (257%), estriol (48.6%), and IGF binding protein-3 (21.1%) were significantly higher in the Chinese than U.S. samples, and cord prolactin was lower (-14.9%). Cord estradiol and IGF-I concentrations did not differ by race/ethnicity. With adjustment for gestational length, maternal age, pre-pregnancy weight, and weight gain, androstenedione (60.5%), testosterone (185%), and IGF binding protein-3 (40.4%) remained significantly higher in the Chinese, whereas the higher estriol and lower prolactin concentrations were attenuated. In addition, estradiol levels became lower in the Chinese (-29.8%) but did not reach statistical significance. Results were generally similar when restricted to first full-term pregnancies, with reduced estradiol concentrations in the Chinese reaching statistical significance after adjustment. These data are consistent with the hypothesis that elevated prenatal androgen exposure could mediate reductions in breast cancer risk. The meaning of the change in findings for estrogens after controlling for factors related to the pregnancy is unclear with regard to explaining international breast cancer differences.