Neurobehavioural development of 35 infants of drug dependent mothers was compared with the development of 37 reference infants as part of a prospective longitudinal research project. Infants of drug dependent mothers had more poor responses than the other children on neurological examination. This difference is significant only when data of infants of drug dependent mothers born at full term are analysed. Two tailed testing indicated that significantly more infants of drug dependent mothers than reference children had electro-encephalograms rated as suspect or abnormal. By the end of the first month the infants of drug dependent mothers tended to be more active, and they had worse scores than the reference children on the neonatal behavioural assessment scale. Analysing data only of infants born at full term, the groups differ significantly on the interactive items. The results of this study show that even after treatment for the neonatal abstinence syndrome, infants of drug dependent mothers seem to differ from comparison children, which could indicate later developmental problems.