Aim: To determine the efficacy of the drug Bosentan, an endothelial receptor antagonist, in patients of secondary pulmonary arterial hypertension having irreversible obstructive pulmonary disease on the basis of improvement in 6 minutes walk test (6MWT) and reduction in Pulmonary Arterial Systolic Pressure (PASP) measured by echocardiography. Methods: This prospective clinical study was conducted over a period of 18 months from October 2015 to March 2017. Patients attending OPD or admitted in the Respiratory Medicine ward with irreversible obstructive pulmonary disease and associated mild to moderate PAH (PASP 25-55 mm of Hg), confirmed by echocardiography, were enrolled. These patients were administered an oral dose of Bosentan 62.5mg twice a day for 4 weeks followed by dose increment to 125 mg twice a day for a duration of 8 weeks. Baseline and 12 weeks values of PASP, 6MWT, SPO2 and spiromateric data were compared. Results: Treatment with Bosentan for 12 weeks in patients with PAH secondary to irreversible obstructive airway disease resulted in a significant improvement in primary end points. PH decreased from 47.54 ± 6.15 mm Hg to 39.19 + 6.17 mm Hg and 6MWD increased from 320.71+52.97 metres to 410+55.74 metres with a mean improvement of 89.29+ 31.78 metres. However, secondary measures of efficacy including oxygen saturation and lung function parameters (FEV1/FVC, FEV1 and FVC) did not show any significant improvement. Conclusion: Bosentan appears to have a significant clinical benefit in patients with PAH secondary to irreversible obstructive pulmonary disease with significant improvement in the 6 MWD and PASP.