The morphological changes in the functional structure of placentas from women who smoke cigarettes during pregnancy have been investigated by quantitative, light-microscopic techniques. It has been demonstrated that the placentas of the smoking mothers were morphologically very similar to the placentas of the nonsmokers. The results only showed a tendency for the placentas in the study group to contain proportionally more nonparenchymal and less parenchymal tissue than the control group, mainly in terms of a relative reduction in the intervillous space volume and peripheral villous mass and surface area. The findings of this study support the hypothesis that the perinatal morbidity associated with cigarette smoking during pregnancy is probably more related to the ischemic and/or toxic effects of several compounds in tobacco smoke, partly on placental function and also directly on the fetus, than to significant alterations in the functional structure of the placenta.