To determine whether it is the smoker or smoking per se that explains the observed reduction in birth weight of infants of mothers who smoked during pregnancy, data collected on 1016 pairs of successive births which occurred between 1946 and 1963 in Washington County, Maryland, were studied. The mean birth weight difference between first and second members of pairs of consecutive live births of the same mother was examined in relation to her smoking habits during both pregnancies. A multiple regression technique was used to simultaneously adjust for the effects of maternal smoking habits during pregnancy, infant sex, maternal age, and birth order on birth weight difference. There were no significant differences in mean birth weight difference among pairs in which the mother smoked throughout both pregnancies, pairs in which the mother was a nonsmoker during both pregnancies, and pairs in which the mother smoked during the second pregnancy of the pair, but not the first. When birth weights of only first members of the pair were studied, the mean birth weight of infants of women who were nonsmokers throughout both pregnancies was higher than that of infants of women who smoked during both pregnancies. Infants of first members of pairs in which the mothers smoked only during the second pregnancy tended to have birth weights which were lower than that of infants of nonsmokers and higher than that of infants of smokers. These findings neither confirm nor deny the hypothesis that the smoker rather than the smoking per se causes a reduction in birth weight.