We assessed neurological status and cognitive, verbal, social and motor skills of 17 children who had undergone cardiac surgery under profound hypothermia, and of seven siblings (controls). None of the subjects had significant neurological impairment. The mean development quotient (DQ) for the patients was 92.5 (in the low-average range) and for the controls was 103 (average). The DQ of one patient (71) was below the normal range. Development of the patients was comparable with that reported by other authors for children with cyanotic congenital heart-disease who have not undergone cardiac surgery. It correlated with the type of congenital heart-disease but not with the duration of hypoxic arrest. It is concluded that profound hypothermia can be used safely during the correction of cardiac defects in infants, without fear of retarding psychomotor development.