Limited information is available on the long-term effects of in utero methadone exposure. This report describes the somatic and neurobehavioral findings of children in the first 18 months of life born to methadone-maintained mothers and to a matched drug-free comparison group of mothers. Findings during the neonatal period were (1) a 75% incidence of moderate-to-severe narcotic abstinence syndrome, (2) a significant incidence of head circumferences below the third percentile, and (3) elevated systolic blood pressure. In follow-up, the methadone children had (1) a significantly higher incidence of otitis media; (2) a significant incidence of head circumferences below the third percentile; (3) neurologic findings of tone discrepancies, developmental delays, and poor fine motor coordination; (4) a high incidence of abnormal eye findings; and (5) significantly lower scores on the Bayley mental and motor developmental indices. These neurobehavioral findings in children of methadone-treated mothers at 18 months of age may be predictors of later learning and behavioral problems.