Young adults with cyanotic congenital heart disease have a high incidence of respiratory and haemostatic problems. Activated neutrophils release vasoactive and chemotactic factors which result in endothelial injury, lung parenchymal damage and the activation of platelets and coagulation pathways. To investigate the contribution of neutrophil activation to morbidity in young adults with cyanotic congenital heart disease, plasma neutrophil elastase levels were measured in 25 cyanotic patients and the results compared to patients with acyanotic heart disease and normal controls. Neutrophil elastase levels were significantly elevated in the group with cyanotic congenital heart disease (P < 0.001). Platelet activation was significantly increased in the patients with cyanotic heart disease (P < 0.001). Platelet aggregation was impaired only in those with haematocrits greater than 0.50 (P < 0.02). Whole blood coagulation, as determined by thrombelastography, was within normal limits. The reason for neutrophil activation in patients with cyanotic congenital heart disease is unclear, but activated neutrophils may contribute to the respiratory and haemostatic problems common to these patients.