The aim of this study was to determine whether cigarette smoking during pregnancy has an adverse effect upon the placenta's capacity for gaseous exchange. Using recently developed stereological techniques, in conjunction with perfusion fixation, computer‐assisted measurements were made on the placentas of 15 non‐smokers, 15 moderate smokers, 15 heavy smokers and 13 ex‐smokers, 7 of whom stopped smoking during the course of the pregnancy. Compared with the placentas of non‐smokers and of those who stopped before pregnancy, it was found that the placentas of smokers and of those who stopped after conception exhibited a reduced capillary volume fraction, and an increased thickness of the villous membrane. Although they must impair gaseous exchange across the placenta, these changes were less severe than suggested by previously published reports. Nonetheless it is clear that in order to prevent these changes women should stop smoking before conception rather than during the course of a pregnancy.