Bosentan is a dual endothelin-receptor antagonist and it is registered for the treatment of pulmonary arterial hypertension (PAH), which records a median life expectancy of 2.8 years. It competitively antagonizes the binding of both endothelin receptor subtypes with high affinity and specificity, and it has been shown to decrease inflammatory reactions and development of fibrosis. The effects of bosentan in children are not well known: in fact no studies have been performed on babies weighting less than 11 kg. We report a premature female baby aging 36 weeks, 24 10 g, who developed a neonatal respiratory distress responsive to ventilatory support due to primary PAR After ventilatory weaning she required chronic medical therapy with desametasone, aldactazide and nifedipine, but the treatment was unsuccessful. When she was 2y 7m, 10.500 g, bosentan therapy 31.25 mg bid was approved by our local Ethic Committee. Pulmonary resistances and right ventricular pressure were evaluated by hyper-O-2 and NO-PG1 tests and both related to transcutaneous O-2 and CO2 pressures and SaO(2). Bosentan therapy was effective to primary PAH.