Background: Pulmonary arterial hypertension is a common cardiovascular complication in Thalassemic patients.  Sildenafil had been shown to have benefits in the treatment of primary hypertension in uncontrolled trials. Although the efficacy of this drug is very convincing, no data is available for thalassemic patients with pulmonary hypertension.Methods: In this prospective, double-blind study, we randomly assigned adult thalassemic patients with pulmonary hypertension to receive either sildenafil (50 mg three time a say (7 subjects)), or placebo (7 subjects) for 4 weeks. The primary endpoints was the changes in the systolic pulmonary arterial pressure (determined by measuring tricuspid regurgitation gradient using echocardiography) after four week treatment period. Secondary endpoints included the changes in functional status and 6-minute walk distance.Results: Patients in both treatment groups had similar baseline characteristics. The change in systolic pulmonary arterial pressure was – 3.457 mmHg, where the minus sign denotes a decrement, with sildenafil, as compare with +2.443 mmHg with placebo, for a treatment difference of -5.9 mmHg. (95% CI, -21.439 to 9.639 mmHg, p = 0.535), indicating that sildenafil was not superior to placebo.  The effects of sildenefil on the secondary end points were not significantly different from placebo.Conclusion: Sildenafil 50 mg three time daily could not demonstrated beneficial effects on systolic pulmonary arterial pressure in thalassemic patients with pulmonary arterial hypertension.