OBJECTIVE To determine whether maternal methadone dosage affects duration and degree of neonatal narcotic withdrawal. METHODS This was a retrospective cohort study of pregnant women with opioid addiction who delivered live-born singletons between April 1990 and April 2001. Inpatient detoxification or outpatient methadone maintenance therapy was offered. Women who had a positive drug screen or whose neonate tested positive for opioids were considered to be supplementing. We evaluated indices of neonatal withdrawal according to the maximum daily methadone dosage in the last week of pregnancy. RESULTS Seventy women with opioid addiction were followed. Median methadone dosage was 20 mg (range 0-150 mg), and 32 infants (46%) were treated for narcotic withdrawal. Among women who received less than 20 mg per day, 20-39 mg per day, and at least 40 mg per day of methadone, treatment for withdrawal occurred in 12%, 44%, and 90% of infants, respectively (P < 0.02). Methadone dosage was also correlated with both duration of neonatal hospitalization and neonatal abstinence score (r(s) =.70 and.73 respectively, both P <.001). Neonates were more likely to experience withdrawal if their mothers were supplementing with heroin, 68% versus 35% (P =.01). Regardless of supplementation, there was a significant relationship between methadone dosage and neonatal withdrawal (P <.05). CONCLUSIONS Maternal methadone dosage was associated with duration of neonatal hospitalization, neonatal abstinence score, and treatment for withdrawal. Heroin supplementation did not alter this dose-response relationship. In selected pregnancies, lowering the maternal methadone dosage was associated with both decreased incidence and severity of neonatal withdrawal.