A randomized clinical trial was conducted to compare the early development of children with d-transposition of the great arteries (N=171) repaired by an arterial switch operation that used either predominantly total circulatory arrest or predominantly continuous low-flow cardiopulmonary bypass. The children assigned to the circulatory arrest group scored lower than those assigned to low-flow bypass on the Bayley Scales of Infant Development at 1 year of age. Responses to parental questionnaires completed when the children were 2.5 years old indicated that the children in the circulatory arrest group, especially those with a ventricular septal defect, also manifested poorer expressive language and were considered to display more internalizing and externalizing problem behaviors. The use of circulatory arrest to protect vital organs during open heart surgery might place children at increased risk of early developmental difficulties, especially in the domains of motor and language function.