1159 mother-infant "pairs" have been studied to examine the inter-relationship of cigarette smoking in the latter half of pregnancy, maternal weight gain, and fetal growth. Non-smokers gained significantly more weight than heavy smokers (greater than 15 cigarettes per day) while light-to-moderate smokers (1-14 cigarettes a day) were intermediate. Birth-weight, length, and head circumference of the infants showed a similar gradient with infants born to non-smokers being heavier, longer, and with larger head circumferences than those born to heavy smokers. Co-variance analysis showed that a large part of the effect of maternal smoking is mediated through maternal weight gain with only a very small additional direct effect on the fetus. This suggests that increasing weight gain in smoking mothers might prevent some of the harmful effects of smoking on fetal growth. A randomised controlled trial of diet supplementation of smoking mothers would seem justified.