The primary purpose of this paediatric study is to provide clinically relevant information on the safety and pharmacokinetic profile of ambrisentan in children with the most common causes of PAH in this age group. The design of the study is also intended to provide information to guide dose selection and supportive efficacy data in this age group. Despite the fact that none of the currently available adult treatments are licensed for use in children <12 yrs, (with the exception of bosentan which was recently approved for use in paediatric population from 2 years of age) they are widely used off label. This study will provide useful prescribing information to the medical community for treating this orphan disease in children in this environment of rapidly changing medical practice.