We did not have information on maternal smoking habits after delivery, but these are likely to be closely associated with smoking habits during pregnancy. Discrimination between the effects of maternal smoking during pregnancy and after birth on infant death therefore remains speculative. If passive exposure to cigarette smoking after delivery explained our findings, the risk of respiratory deaths would most likely be increased among children of smokers compared with children of nonsmokers. However, children of smokers were not at increased risk of respiratory deaths compared with children of nonsmokers. Furthermore, the distribution of causes of infant deaths showed that exposure to tobacco smoke in utero apparently increased the risk of death due to a variety of causes, which may indicate that the harmful effect of exposure to tobacco smoke is not restricted to a single organ.