Eighty one children, comprising 51 with tetralogy of Fallot and 30 children with transposition of the great arteries (TGA) were assessed using the Wechsler intelligence scale for children--revised, and a battery of neuropsychological measures. They were compared with a group of 33 children who had surgery for ventricular septal defect. All children were aged over 10 years when reviewed and were in good health, attending normal schools. No significant negative correlation was found between any component or subtest of the IQ scores and operating age. There was no evidence of a detrimental effect of older age at operation in the children who had cyanotic heart disease as assessed by neuropsychological measures. Delaying surgery for children with TGA or tetralogy of Fallot does not appear to adversely affect their intellectual development. This finding may provide reassurance in cases where surgery has to be delayed for medical, social, or economic reasons.