The present investigation evaluated the effects of maternal smoking during pregnancy on the growth of lung function during the first 18 mo of life in 159 infants who were part of a longitudinal study of the effects of maternal smoking on respiratory health. Infant pulmonary function was assessed at 2 to 6 wk of age and at 4 to 6, 9 to 12, and 15 to 18 mo of age by partial expiratory flow-volume curves and helium dilution FRC. Maternal smoking was assessed by standard questionnaire and urine cotinine measurements. Maternal smoking during pregnancy was associated with a reduction of 9.4 +/- 4.3 ml (p = 0.029) for FRC and 33 +/- 12.3 ml/s (p = 0.008) reduction for flow at FRC (VFRC) after controlling for the effects of growth (length). The effect of maternal smoking was greater for female infants than for male infants. At 1 yr female infants exposed in utero were predicted to have a 16% reduction in VFRC compared with 5% for male infants. No sex difference was seen for FRC or VFRC/FRC. Exposure to environmental tobacco smoke in the postnatal period was not significantly related to reduced FRC or VFRC. These data provide further evidence that maternal smoking during pregnancy may play a greater role than postnatal and childhood exposure on the observed effects on lung function in children.