We randomly assigned 40 patients scheduled for surgical correction of congenitally malformed hearts under cold blood cardioplegic arrest to postconditioning or control treatment. Postconditioning was performed by two cycles of 30 seconds ischaemia and 30 seconds reperfusion using aortic reclamping, and declamping started 30 seconds after cardioplegic arrest. We assayed creatine kinase-MB, troponin I, transcardiac release of lactate and neutrophil counts.