Background: In patiens with congenital heart defects, pulmonary arterial hypertension (PAH) is an important factor for morbidity and mortality. Congenital systemic to pulmonary shunts represent an important aetiology of PAH in children. The elevated pulmonary artery pressure and pulmonary vascular resistance can cause a reversed or bidirectional shunt, known as Eisenmenger syndrome. The congenital intra-cardiac communication may serve as a "safety valve", preserving cardiac output, however at the expence of cyanosis, exercise limitation and increased cardiac work. Today there is no known efficient treatment for this condition. However, in different studies, several new drugs have improved exercise capacity and cardiopulmonary hemodynamics in patients with PAH. The endothelin-receptor antagonist bosentan is one of them. Endothelin-1 is a potent vasoconstrictor and smooth-muscle mitogen. The present trial was done in effort to evaluate effects and develop non-invasice markers of physiologic and hemodynamic changes during bosentan treatment in children with Eisenmenger syndrome.Methods: In this open label, uncontrolled, prospective study, 14 children with Eisenmenger syndrome were to receive 1 mg/kg of bosentan twice daily for 4 weeks, followed by 2 mg/kg twice daily for 12 months. The primary end points were the change in symptoms, oxygen saturation and exercise capacity.Results: Some of the patients had an improvement of their symptoms. However the results from the blood samples and the oxygen saturation measurements, showed less convincing results.Conclusions: Larger (RCT) trials show that bosentan is beneficial in patients with pulmonary arterial hypertension in adults with related conditions. The results of the present trial is not conclusive with respect to the treatment effect of bosentan in children.