BACKGROUND: Pulmonary arterial hypertension (PAH) is a chronic, debilitating disease affecting millions of adults worldwide. With improved knowledge on PAH and better management, long-term survival in patients has significantly increased in the past 20 years. Bosentan is a nonselective, dual endothelin receptor antagonist used in the treatment of PAH. While the drug has already been established to improve exercise capacity and patient survival globally, no study has investigated its clinical effectiveness and safety among Filipino patients yet. A post marketing study was conducted to determine the efficacy and safety of bosentan (125 mg administered twice daily) among adult Filipino patients with PAH. METHODS: A non-randomized, non-comparative, open-label trial was conducted involving adult patients at a tertiary government hospital in Metro Manila. Study duration was from March to September 2012. Primary end points of the study were patients' response to efficacy and safety. RESULTS: A total of 14 patients were enrolled in the study and 13 included in the analysis. Mean age of the participants was 34 ± 11.54 years. Remarkable changes were observed for 6WMD and small improvements noted for Borg dyspnea index and CPET. There was no difference between baseline and 12th week WHO functional classification. FEVl /FVC, MVV, RV /TLC and sRaw showed modest improvement; there was a notable difference in the systolic PAP vs baseline; PVR, PVRI, SVR and SVRI demonstrated the largest changes via cardiac catherization and iloprost. Four patients experienced at least one serious adverse event, with one reported as suspected unexpected serious adverse reaction. Out of 13 patients, 1 O (76.9%) considered bosentan as effective while 11 (84.6%) considered it safe. CONCLUSION Bosentan improved exercise capacity, pulmonary function and cardiopulmonary hemodynamics among study participants. The drug is generally well-tolerated and effective. Bosentan is among the useful options for treatment of adult Filipino patients with PAH.