Between January 1985 and March 1989 we retrospectively observed multiple system organ failure (MSOF) in 16 of 460 children (3.5%) who underwent cardiac surgery with cardiopulmonary bypass for congenital heart disease. MSOF was arbitrarily defined as a clinical entity with failure of two or more vital organ systems associated with high fever, thrombocytopenia, and cardiocirculatory insufficiency and occurring within the first postoperative week. In 13 children the first clinical manifestations of MSOF were evident on the first post-operative day and in the other 3 on the second or third post-operative day. All children showed acute renal failure, acute hepatic failure, high fever, and thrombocytopenia. Most of them showed respiratory insufficiency and neurological involvement. Seven of the 16 children died. Four of the 9 surviving patients had neurological sequelae still present 6 months after the operation, and the others recovered completely.