AMB-320/321-E was designed to provide long-term, controlled monitoring of pulmonary arterial hypertension (PAH) patients treated with ambrisentan (AMB) in order to properly define the adverse event profile associated with this endothelin receptor antagonist (ERA), including the incidence and severity of elevated serum liver function tests (LFTs). In addition, this study continued the efficacy assessments of the previous studies, examined long-term AMB treatment success, and compared long-term survival of subjects treated with AMB to the NIH registry of patients with PAH.