During the years 1971--1974, 230 infants born to drug-dependent women and 33 infants born to ex-addicts were studied. Heroin abuse declined while methadone usage increased during those years. Compared to heroin abuse, methadone maintenance treatment during pregnancy was associated with more consistent prenatal care, more normal fetal growth and reduced fetal mortality. Meconium staining of amniotic fluid was increased in the heroin and heroin-methadone groups; this was not associated, however, with an increase in meconium aspiration or a reduction in Apgar scores. Of special note was the equally severe intrauterine growth retardation of infants of former heroin addicts who were free of narcotic use during pregnancy. Neonatal withdrawal from methadone appeared to be more severe than from heroin, as judged by amount of medication required to control symptoms and duration of treatment. In all groups, central nervous system signs were the most common manifestations of withdrawal. Severity of withdrawal did not correlate with late pregnancy maternal methadone dosage. Neonatal seizures occurred in 1.5% of the heroin group and 10% of the methadone group. Discharge of an infant to a parent rather than to an alternate care-taker was more likely if the mother was enrolled in a methadone treatment program. Methadone maintenance programs appear to offer significant therapeutic benefits, balancing the untoward effects of the drug on the newborn infant.