The primary purpose of this long term paediatric study is to provide clinically relevant information on the long term safety of ambrisentan in children with the most common causes of PAH in this age group. This study is only open to patients who have participated in Study AMB112529, A randomized, open label study comparing safety and efficacy parameters for a high and a low dose of ambrisentan (adjusted for body weight) for the treatment of pulmonary arterial hypertension in paediatric patients aged 8 years up to 18 years, and in whom continued treatment with ambrisentan is warranted.