A 6-month (24-week), randomized, open label evaluation of the safety, tolerability, and efficacy of a high and low dose ambrisentan (adjusted for body weight) treatment group in subjects aged 8 years up to 18 years with pulmonary arterial hypertension (PAH). An additional objective is to determine the ambrisentan population pharmacokinetics in the paediatric population. The study will include a screening/baseline period and a treatment period. The treatment period will be 24 weeks or until the subject's clinical condition deteriorates to the point that alternative/additional treatment is necessary. Patients who participate in the study and in whom continued treatment with ambrisentan is desired will be eligible to enrol into a long term follow-up study. The primary comparison will be the safety and tolerability of the two ambrisentan dose groups (Low vs. High) in the paediatric PAH population The secondary comparison will be the change from baseline for the efficacy parameters between the two treatment groups.