Infants of mothers who used methadone, heroin, cocaine and other drugs during pregnancy were compared with a group of control infants over the first 30 mths of life. Results of repeated examinations with the Bayley Scales of Infant Development showed a difference between the groups in mental development. No differences were found in a non-verbal version of the mental scale, but infants of drug dependent mothers seem to have specific difficulties in early language development. In psychomotor development, in behaviour during testing and on a questionnaire on activity, infants of drug dependent mothers did not differ from the comparison children.