OBJECTIVES: Primary pulmonary hypertension (PPH) is a disorder with limited treatment options. Sildenafil, an oral phosphodiestrase type-5 (PD-5) inhibitor and a pulmonary vasodilator, is likely to be beneficial in patients with primary pulmonary hypertension. we hypothesized that a single dose of Sildenafil could acutely reduce peak pulmonary artery pressure and improve echocardiographic diameters of right heart. METHODS: We studied 12 consecutive patients with PPH (10 patients with New York Heart Association functional class ²²², and 2 patients with functional class ²²). After initial echocardiographic evaluation, a 100 mg oral single dose of Sildenafil was added to previous medication of all patients and 1 hour after that echocardiograpdic parameters was measured again, and the results were compared. RESULTS: Peak pulmonary artery pressure and pulmonary acceleration time (PAT) significantly improved 1 hour after 100 mg oral single dose of Sildenafil (p <0.005, 95% CI 5.41-22.93 and p = 0.005 95% CI -12.89- -2.95 respectively). In addition although the right heart dimensions (right atrium & ventricle) showed a trend toward improvement, but were not statistically significant (p = 0.135, p = 0.08 respectively). CONCLUSION: These results suggest that Sildenafil has a acute significant improvement effect on PAP and PAT in patients with PPH.