The effects of parental smoking on IgE and IgD levels in cord serum and subsequent infant allergy were investigated in an unselected series of 186 European newborn infants. Maternal smoking caused a significant rise in both IgE (p less than 0.05) and IgD (p less than 0.05), a finding that was most apparent in newborn infants with a negative biparental history of allergy (p less than 0.025 and p = 0.005, respectively). Furthermore, newborn infants of nonallergic parents had a more than threefold (p less than 0.01) higher incidence of elevated cord IgE (greater than or equal to 1.20 IU/ml) and a fourfold (p = 0.005) higher risk of developing definite or probable atopic disease before 18 months of age if the mother smoked than if she did not. Paternal smoking did not influence, in whatever the subgroup, cord IgE or subsequent infant allergy but increased cord IgD (p less than 0.001) among newborn infants with a negative family history even after controlling for maternal smoking (p less than 0.04). These results suggest that parental smoking in some way affects the fetal immune system, probably via substances in tobacco smoke. Maternal smoking in particular appears to exert a pronounced effect on the IgE system already in fetal life, predisposing even "low-risk" infants to subsequent sensitization, probably in synergy with a later acquired mucosal damage that would facilitate penetration of foreign matter. Pregnant women and mothers should be encouraged to try and give up smoking that might help to prevent allergic disease in their infants.