Data were analyzed from a large prospective study to try to determine whether or not women's smoking during pregnancy affects their children's mental development. Many confounding variables were controlled by multiple regression analysis and by intrapair comparisons of siblings whose mothers had smoked during one but not in the other of the two pregnancies. Hyperactivity, short attention span, and lower scores on spelling and reading tests were more frequent for children whose mothers had smoked throughout pregnancy. The cognitive abnormalities were mild, with achievement test scores only 2 to 4% lower in children whose mothers had smoked during pregnancy. The behavioral abnormalities in children of smokers were associated with elevated neonatal hemoglobin levels and low birth weights, suggesting that fetal hypoxemia possibly may contribute to the genesis of behavioral abnormalities.