We have studied the neurodevelopmental outcome of 104 consecutive unselected children who underwent open-heart surgery from 1987 through 1989. Survivors had formal neurologic and psychometric examinations after 2 years of age. Mean IQ was 90, and 78% had scores above 70. Cerebral palsy occurred in 22%. Deep hypothermia for longer than 45 minutes was associated with IQ less than 85 (P < .001) and later cerebral palsy (P = .02). Those less than 1 month old at operation had a median IQ of 96, and 25% had cerebral palsy. Median IQ for survivors of hypoplastic left heart syndrome was 66, only one had an IQ above 70, and 57% had cerebral palsy. Median IQ for transposition of great arteries was 109, only one was less than 85, and all had normal neurologic examinations. Those between 1 and 6 months of age at operation had a median IQ of 93, with 64% above 85, and 5% had cerebral palsy. Those older than 6 months had a median IQ of 99, with 70% above 85, and 13% had cerebral palsy. For infants less than 1 month old at operation, a strong association existed between outcome, type of lesion, and duration of deep hypothermia (P < .01), although not in all cases. In those older than 1 month at operation, no association existed between outcome and any study variable. Although the majority of children have an uneventful outcome after open-heart surgery, a significant morbidity exists. This is related to several factors, including type of lesion and duration of hypothermia, particularly in neonates; preoperative congenital and acquired lesions; and possible perioperative cerebrovascular events.