Studies concerning smoking during pregnancy and cognitive development of the child were reviewed. Some studies tend to support the existence of an association between smoking during pregnancy and a less optimal cognitive development of the child. The meaning of these relationships remains obscure, since it cannot be assumed that they are due to maternal cigarette smoking: they could be due to different characteristics of smokers or non-smokers. In the older studies, potential confounding factors were not adequately taken into account and in some recent studies, the observed difference was no longer significant after taking into account the maternal educational level and the family stimulation of the child. Behavior problems are associated with smoking during pregnancy in nearly all studies but the same problem of confounding factors remains. It is thus difficult to conclude presently from these studies if there is a causal relationship between prenatal exposure to tobacco smoke and cognitive development.