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Background and Aim: Plasma brain natriuretic peptide (BNP) level increases in proportion to the degree of right ventricular dysfunction in pulmonary hypertension. The aim of this study was to evaluate outcome of treatment and the prognostic value of N-terminal brain natriuretic peptide (N-TproBNP) in patients with pulmonary arterial hypertension (IPAH). Materials and Methods: This study was a longitudinal trend study. It was conducted in 2011 till 2013 on 20 patients with PAH at Massih Daneshvari hospital, who were treated with intravenous iloprost for 12 weeks. Before, during and after treatment WHO functional class, six minute walking test (6MWT), pulmonary artery pressure (PAP) and right ventricular pressure (RVP) in echocardiography and plasma NT-ProBNP level was measured. Results: Findinges showed that iloprost infusion improved exercise tolerance and functional class and decreased PAP and RVP in patients with IPAH. The level of NT-ProBNP had decreased in 75% of patients and its mean had decreased significantly to 1409 pg/ml (P=0.009). Conclusion: Iloprost is an effective therapy for patients with IPAH. Furthermore, Plasma levels of NT- ProBNP were decreased proportionally to elevate in functional class and 6MWT distance and decrease in PAP and RVP. Measurement of NT-ProBNP is actually advised for the routine assessment and for the clinical decision making of PAH patients