Behavioral characteristics of infants of methadone-treated and nonaddicted women were studied during the neonatal period and at three, six, and 12 months of age. Several Brazelton Neonatal Behavioral Assment Scale measures differentiated between groups. Major differences occurred in irritability of the central nervous system, and these measures appeared able to predict severity of withdrawal. The increased tremulousness of addicted infants remained through at least the first month of life. Mental and motor development was within normal limits in both groups throughout the year. However, the addicted infants showed a progressive decline in psychomotor performance whereas nonaddicted infants' scores remained stable.