Mothers' and fathers' birthweights are each significantly related to their infants' birthweight, even after control for 31 potentially confounding variables. The relationship is stronger for mothers and daughters than for other parent-offspring pairs. Parents' birthweights together account for 2% of the variance in sons', and 5% of the variance in daughters' birthweights. There are interactions of maternal birthweight with maternal adult weight, pregnancy weight gain, and parity; the predicted effect of maternal birthweight on infant birthweight is enhanced when the mother weighs more, gains more weight during pregnancy, or is multiparous. Race and parents' drinking and smoking also modify the predicted effect of parents' birthweights. The correlation between mother and offspring birthweights has been attributed to the mother's own uterine experience and its effect on her later reproductive performance. However, the significant relationship of father and infant birthweights reported in this paper suggests that the fetal genes also play some role in determining size at birth.