• The impact of maternal opiate dependency on birth weight and head circumference of offspring was studied in 150 mother-infant pairs and 150 controls. Mean birth weight and head circumference of drug-dependent offspring were reduced symmetrically compared with drug-free controls of similar socioeconomic status. Although mean gestational ages and rates of prematurity were similar, intrauterine growth retardation occurred in 20% of passively addicted infants vs 4% of controls. A longer period of methadone maintenance was associated with longer gestation and increased birth weight, while higher methadone dosages were associated with higher birth weights and larger head circumferences. Smoking was associated with reduced birth weight by an average of 160 g per pack of cigarettes smoked per day; longer duration of smoking was associated with reductions in both birth weight and head circumference. These data suggest therapeutic strategies to improve these fetal growth characteristics and perhaps improve neurobehavioral outcome of drug-dependent newborns.