Linked birth certificate and infant death certificate data from Missouri for 1979-1983 were used to explore the association of maternal smoking with age and cause of infant death. The data included 305,730 singleton white livebirths, of which 2,720 resulted in infant deaths. Using multiple logistic regression to control for the confounding effects of maternal age, parity, marital status, and education, the authors found that smoking was associated with both neonatal and post-neonatal mortality and with each cause of death except congenital anomalies. The adjusted odds ratio for smoking was higher for postneonatal deaths than neonatal deaths and was particularly high for two causes: respiratory disease (odds ratio = 3.4) and sudden infant death syndrome (odds ratio = 1.9). A moderate odds ratio (about 1.4) was found for causes attributed to the International Classification of Diseases, 9th Revision Perinatal Conditions Chapter. Although the associations for neonatal deaths and perinatal conditions were partially attributable to the effect of maternal smoking in lowering birth weight, virtually none of the excess respiratory mortality and sudden infant death syndrome mortality among the offspring of smokers was attributable to birth weight differences between the infants of smokers and nonsmokers. This suggests that respiratory deaths and sudden infant death syndrome deaths may be related to the effect of passive exposure of the infant to smoke after birth.