Effects of cardiopulmonary bypass (CPB) and hypothermic circulatory arrest on brain morphology were evaluated by computed tomography (CT). Of 57 children undergoing cardiac operations, 45 (4.5 +/- 2.8 years of age) were operated upon with the use of CPB with high-flow, mildly hypothermic perfusions. Twenty-seven of them were perfused with bubble oxygenators and 18 with membrane oxygenators. In the bubble oxygenator group, all 14 with 20 mu filters in the arterial line showed no postoperative CT changes, whereas four of 13 (31%) with 40 mu filters or without filters showed decreases in brain mass on CT scans. Three of these four patients underwent perfusion for more than 80 minutes. There were no CT changes in the membrane oxygenator group. Twelve infants (10.4 +/- 4.5 months of age) were operated upon with the aid of deep hypothermia and circulatory arrest (core temperature below 20 degrees C). Ten of 12 who had circulatory arrest for less than 60 minutes showed no CT changes, but two infants who had circulatory arrest for more than 60 minutes showed changes similar to those described above. All six children with CT changes had no clinical manifestation of the brain damage, and their CT abnormalities recovered within 6 to 11 months after operation. The specific cause of these changes remains undetermined, but microemboli or hypoxia during operation could be implicated.