The associations between parental smoking habits during pregnancy, family history of allergy, and IgE levels in cord-serum IgE (cIgE) were investigated prospectively in 1251 infants. Mean cIgE levels were similar in infants with positive and negative family history of allergy (0.50 and 0.54 kU/l, respectively). In infants with a positive family history of allergy whose mothers smoked more than 10 cigarettes a day, however, the mean cIgE level was significantly higher (0.78 kU/l) than that in infants of nonsmoking mothers (p = 0.011). Similarly, the mean cIgE level was significantly higher in boys (0.61 kU/l) than in girls (0.51 kU/l) of nonsmoking parents (p = 0.03). However, in infants of smoking parents, there was no significant difference in the mean cIgE level between boys and girls. A total of 178 (14%) mothers and 477 (38%) fathers were current smokers, and in 128 (10%) cases, both parents were active smokers. The mean cIgE level tended to be slightly higher in infants of smoking mothers, especially when the mother consumed more than 10 cigarettes a day (0.63 vs 0.54 kU/l) (p > 0.05). Thus, cIgE levels were higher only in genetically prone babies whose mothers consumed more than 10 cigarettes a day.