Using a radio-isotope method, the perfusion through the intervillous spaces of the placenta was recorded in 23 pregnant smokers and 23 pregnant non-smokers between 33rd and 37th week of gestation. All women were normotensive and the two groups were comparable in all essential variables. As compared with that in the nonsmokers, the perfusion was significantly lower in the smokers and the infants borne by the smokers had a significantly lower birth weight. In both groups there was a positive correlation between the perfusion values and the birth weights. Because we found a slight, chronic reduction in the blood flow through the intervillous space of the placenta in pregnant smokers, the results suggest that the changes caused by smoking are rather permanent. We find it well-founded to advise pregnant women against smoking.