A variety of in utero factors have been associated with risk of adult cancers, particularly birth weight, toxemia, and gestational age. These factors are thought to reflect hormonal exposures during pregnancy. We hypothesized that the prenatal hormonal milieu may explain part of the variation in cancer rates across ethnic groups, for example, the higher incidence of breast cancer in the Caucasian compared with Hispanic women and the higher incidence of prostate and lower incidence of testicular cancers among African-Americans compared with Caucasians. We measured hormones in early pregnancy blood samples from three ethnic groups in a health care plan in Boston, MA. Mean levels of androstenedione, testosterone, estrone, and prolactin were significantly lower in Caucasian women compared with Hispanic women. Although not statistically significant, estradiol levels were lower in Caucasian compared with Hispanic or African-American women. Concentrations of androstenedione, testosterone, and progesterone were notably higher in African-American compared with Caucasian or Hispanic women. These data are consistent with hypotheses that in utero hormonal exposures may explain some of the ethnic group differences in cancer risk.