A large data set was used to examine the possibility that maternal smoking during pregnancy causes low birth weights by reducing maternal appetite, eating, and weight gain. As always, birth weight distributions shifted downward as maternal smoking level increased. Maternal weight gain distributions, on the other hand, were the same for smokers and nonsmokers. Within each level of maternal weight gain, from less than five pounds to over 40 pounds, the more the mothers smoked the greater was the percentage of neonates weighing less than 2,500 grams. This evidence supports a direct effect of maternal smoking on birth weight, possibly due to the hypoxic effects of carbon monoxide, rather than one mediated through eating. Efforts to prevent or reduce smoking should have greater benefits for mother and child than would efforts to increase food intake among pregnant women who smoke.