BACKGROUND Congenital heart disease (CHD) is associated with complex coagulation abnormalities. Platelet aggregability has not been investigated in detail in children with acyanotic and cyanotic malformations undergoing open heart surgery. The method of whole-blood multiple electrode aggregometry (MEA) appears suitable for rapid platelet analysis in children, for example, because of small sample volumes. We investigated perioperative evolution of platelet aggregation by means of MEA in children with CHD. METHODS Fifty children with acyanotic or cyanotic malformations were included in a prospective observational study. Laboratory testing was assessed before anaesthesia, and during and after surgery until the fifth postoperative day. MEA was performed in hirudin-anticoagulated blood using adenosine diphosphate (ADP), arachidonic acid, and thrombin receptor-activating peptide for platelet activation. Surgical variables, bleeding volumes, and transfusion requirements were documented during hospital stay. RESULTS Mean platelet count was within the normal range in all patients with no intergroup differences. Before surgery, aggregation to all agonists was within the age-adjusted normal range in cyanotic children and was statistically significantly higher compared with acyanotic children. Platelet aggregation decreased significantly during surgery in both groups followed by a slow recovery not reaching baseline levels. Bleeding and platelet transfusions were higher in the cyanotic group. Transfusion requirements correlated with ADP-induced platelet aggregation. CONCLUSIONS These results indicate higher blood loss, despite better platelet aggregation in cyanotic patients compared with acyanotic patients. MEA alone might not be suitable for predicting increased perioperative blood loss.