Bosentan in Children With Pulmonary Arterial Hypertension

Inclusion Criteria: * Signed informed consent by the parents or the legal representatives. * Males or females \>= 2 and \< 12 years of age. * Idiopathic PAH or familial PAH diagnosed by right heart catheterization (Clinical classification of pulmonary hypertension, Venice 2003). * World Health Organization (WHO) functional class II or III. * Oxygen saturation (SpO2) \>= 88% (at rest, on room air). * PAH treatment-naïve patients or patients already treated with either: * Bosentan monotherapy * Intravenous epoprostenol monotherapy * Intravenous or inhaled iloprost monotherapy * Combination of bosentan and intravenous epoprostenol * Combination of bosentan and intravenous or inhaled iloprost. * All patients should start the study drug (bosentan pediatric formulation) at 2 mg/kg twice daily (b.i.d.), whether or not they were previously treated with bosentan. * PAH therapy stable for at least 3 months prior to Screening. * Stable treatment with calcium channel blockers, if any, for at least 3 months prior to Screening. * Patient's PAH condition stable for at least 3 months prior to Screening. Exclusion Criteria: * PAH associated with conditions other than idiopathic or familial PAH. * Non-stable patients, e.g., history (in the last 3 months prior to Screening) of recurrent syncope, or signs and symptoms of non-compensated right heart failure. * Need or plan to wean patients from intravenous epoprostenol, or intravenous, or inhaled iloprost. * Body weight \< 4 kg. * Systolic blood pressure \< 80%, the lower limit of normal range, according to age and gender. * AST and/or ALT values \> 3 times the upper limit of normal ranges. * Moderate to severe hepatic impairment, i.e., Child-Pugh Class B or C. * Hemoglobin and/or hematocrit levels \< 75% of the lower limit of normal ranges. * Pregnancy. * Known intolerance or hypersensitivity to bosentan or any of the excipients.