Data on 178 term and 34 preterm infants born to methadone-maintained mothers were analyzed to assess the effects of neonatal opiate abstinence in infants of varying gestational ages. More mothers in the term group (79%) than in the preterm group (53%) had abused other drugs during pregnancy (p less than 0.001). Mean (+/- SD) gestational age was 39.5 weeks +/- 1.4 for term infants and 34.3 weeks +/- 2.6 for preterm infants. On the basis of a semiobjective symptom scoring scale, term infants had more severe abstinence symptoms and more prominent central nervous system manifestations than preterm infants. The severity of abstinence symptoms correlated with maternal methadone dosage in both term and preterm infants. Maternal multiple drug abuse (e.g., heroin, cocaine) did not influence severity of abstinence symptoms in either group. More term infants (145/178) than preterm infants (20/34) required treatment for these symptoms (p less than 0.005). In 13 of 178 term infants, compared with 1 of 34 preterm infants, abstinence-related seizures developed. Peak severity occurred 1 to 2 days earlier in term than in preterm infants. A less severe abstinence syndrome in preterm infants may be due to (1) developmental immaturity of either dendritic ramifications, specific opiate receptors, or neurotransmitter function, or (2) reduced total drug exposure during the intrauterine period.