The correlation of maternal and paternal physical characteristics to the weight of the newborn infant was investigated. It was found that a different anthropomorphic variable best explained the outcome in each group when mothers were divided by weight for height. In women of low weight for height, weight gain was most significantly associated with birth weight (P less than .01); and in women of intermediate weight for height, the significant variable was maternal prepregnancy weight (P less than .01); in women of high weight for height the outcome was best explained by maternal height (P less than .05). The model accounted for 17, 21, and 32% of the variance in birth weight in the 3 weight-for-height groups, respectively. It appears that the relative value of genetic and environmental factors may be different depending on the body type of the mother. It is postulated that other genetic and environmental factors, including smoking, alcohol ingestion, and exact gestation age, may be of significance. The conclusions of this study differ from those of previously reported studies. Possible explanations include variable population homogeneity and different socioeconomic groups studied as well as the obscuring of biologically significant variables if outcomes in subjects with different body types are not analyzed separately.