In epidemiologic studies of perinatal exposures, birth weight has been proposed as a proxy variable for intrauterine estrogen exposure. To assess the validity of this assumption, we performed analyses of the association between estriol levels in 188 women in the 17th, 25th, 33rd, and 37th weeks of pregnancy and the birth weights of their infants. We found a general increase in mean cumulative estriol dose with increasing birth weight category throughout pregnancy. In late pregnancy, mean pregnancy estriol level of mothers of infants in the highest birth weight category (>4,500 gm) was twice as high as that of mothers of infants in the lowest category (<2,500 gm), 775 nmol/liter and 392 nmol/liter, respectively. Smoking lowered the maternal estriol levels by 20% or more throughout pregnancy. With smoking and birth weight included in a regression analysis, maternal age, placental weight, and infant ponderal index did not add any explanatory power to the model. Our data suggest that, on an aggregate level, birth weight can be used as a proxy variable of intrauterine estriol exposure.