Smoking during pregnancy clearly causes intrauterine growth retardation, but its effects on later growth, if any, are not clear. This study examined 204 newborn infants whose mothers had smoked during pregnancy and 204 control infants. Evaluation was repeated after 1 and 2 years. Term infants were delivered after 37 weeks gestation. All these infants were singleton births, had 1- and 5-minute Apgar scores of 7 or greater, and lacked signs of distress. The median number of cigarettes smoked each day was 10. Maternal smoking was associated with significant effects on growth over time, including body weight, length, and head circumference. Infants whose mothers smoked 1 to 9 cigarettes a day had anthropometric values similar to those of control infants, but significantly retarded growth was noted when the mother smoked 10 or more cigarettes a day. Subsequently, weight retardation became less evident and head circumference remained stable. Retarded body length increased, however, even in children whose mothers smoked fewer than 10 cigarettes a day. On multivariate analysis, both maternal smoking during pregnancy and a low educational level had significant negative effects on growth over time. The infant's feeding status, maternal body mass index at the start of pregnancy, age, and parity did not significantly influence growth. At 2 years of age, significantly more children in the smoking group had a body length below the fifth percentile, regardless of the level of smoking.