A study on dietary and smoking behavior during pregnancy was performed between January, 1976, and September, 1979, at the St. Antoine Maternity Hospital in Paris. In addition to the routine clinical examinations at the third, sixth, eight, and ninth months of pregnancy, the women were systematically questioned about their dietary and tobacco habits. No dietary advice was given by the dietitians at any time, nor was there special counseling against smoking. Among 534 women who were followed up, 200 (37%) were smokers before pregnancy. At the sixth month, half of them had stopped smoking and the others had reduced their consumption. The analysis showed that the mean caloric intake and the gain in weight were the highest in the women who continued to smoke and the lowest in nonsmokers. Intermediate results were found in smokers who stopped. Birth weight was, on the average, 70 gm lower among smokers throughout pregnancy, compared with the other two groups, but this difference was not significant. These findings are consistent with the hypothesis that the negative effects of smoking on the fetus could be compensated for, to a great extent, by extra food.