It is hypothesised that ambrisentan may provide benefit to subjects with inoperable chronic thromboembolic pulmonary hypertension (CTEPH), where currently no proven or licensed treatment options exist. This Phase III, randomized, double-blind placebo controlled parallel group, 16 week study will compare the safety and efficacy of ambrisentan 5 milligrams (mg) versus placebo in subjects with inoperable CTEPH. The study will enrol 160 subjects, to assure at least 72 evaluable subjects per treatment arm, based on 10% drop-out rate.