Pulmonary arterial hypertension is a rare but debilitating disease, resulting in progressive deterioration that is characterised by an increase in pulmonary vascular resistance. The most effective therapy to date has been continuous intravenous infusion of epoprostenol, a relatively complicated therapy with risk of sepsis. Other options are limited in small children with severe disease. Bosentan, or Tracleer, an endothelial receptor antagonist, has shown promising results in adults, but little is known about its efficacy in children. Aims: To examine the safety and efficacy of Bosentan in children with pulmonary arterial hypertension. Characteristics of patients treated: We treated 25 children aged between 10 months and 16 years with Bosentan for periods from 2 to 16 months, with a mean of 7 months. In 13 patients, the pulmonary hypertension was primary and secondary in the other 12. Mean weight at initiation of treatment was 21.8 kg, and the patients were treated with a dose of 15–125 mg, with a mean of 45 mg. All patients had evidence of right atrial and right-ventricular hypertrophy, with three-fifths having evidence of strain. Echocardiograms showed tricuspid regurgitation and right-ventricular hypertrophy in all, and impaired right-ventricular function in 44%. Pulmonary arterial pressure was equal to or greater than systemic in 76%. Pulmonary vascular resistance was elevated in all patients, with measurements of more than 11 Wood units in 84%. Pulmonary vascular resistance did not fall in response to vasodilators, including nitric oxide in 92%. 5 patients were already on Epoprostenol at an average dose of 45 ng/kg/min. Results: Most children showed clinical improvement as indicated by improved exercise tolerance, and a beneficial shift in the classification of the New York Heart Association (see Figure below). Rescue Epoprostenol therapy was needed in 3 children after 8 months of treatment with Bosentan. No side effects have been reported so far. Conclusion: Chronic administration of an oral rather than an intravenous drug is an attractive option, particularly in children. Although our limited experience with Bosentan in treating severe primary and secondary pulmonary hypertension in young children is encouraging, one needs to monitor these patients carefully. Our current indication for Bosentan is, first, for patients in the second or third classes of the New York Heart Association at presentation, second, high pulmonary vascular resistance which is not reactive to acute vasodilators, and third, patients in whom treatment with Epoprostenol is not an option.