Testosterone (T) and estradiol (E2) concentrations were determined and correlated with beta human chorionic gonadotropin (beta-HCG) concentrations in 43 paired maternal and cord sera (22 female and 21 male infants). Mean (+/- SD) maternal E2 concentrations were significantly (P less than .005) higher when the sex of the fetus was male than when the sex of the fetus was female (20.6 +/- 3.9 vs 13.5 +/- 3.2 ng/ml). Maternal T concentrations were not significantly different when related to the sex of the fetus (males, 114.8 +/- 60.7 vs females, 113.8 +/- 54.5 ng/100 ml, P greater than .1). Regression analysis did not show a significant correlation between maternal T or E2 concentrations and maternal beta-HCG concentrations. Mean cord serum T and E2 concentrations of male infants were significantly greater than that of female infants (T, 38.8 +/- 8.5 vs 25.8 +/- 7.1 ng/100 ml, P less than .005; E2, 9.1 +/- 3.3 vs 6.6 +/- 2.0 ng/ml, P less than .005). Regression analysis showed a significant (P less than .005) correlation between cord beta-HCG concentrations and E2 concentrations for male infants (r = .7) and female infants (r = .6). A significant correlation between cord beta-HCG concentrations and T concentrations was found for male infants (r = .5; P less than .01) but not for female infants (r = .3; P greater than .05). There was no correlation between maternal and infant E2 concentrations (males, r = .3, P greater than .05; females, r = .3, P greater than .2) or T concentrations (males, r = .02, P greater than 0.4; females, r = .06, P greater than .3). These data (1) confirm the sex difference in cord serum T and E2 concentrations, (2) indicate that the lower beta-HCG concentrations in mothers of male infants are associated with E2 concentrations which are greater than those in mothers of female infants, and (3) are consistent with an influence of beta-HCG on fetal T and E2 secretion.