Background: Pulmonary arterial hypertension (PAH) is a progressive and fatal disease. Sildenafil is a type 5 phosphodiesterase inhibitor and pulmonary vasodilator. Therefore, we hypothesized that sildenafil would improve distance walked in 6 minutes and hemodynamics in children with PAH.Patients and methods:In this case-control study,we assigned 30 patients with symptomatic pulmonary arterial hypertension (either primary or secondary)to sildenafil orally three times daily for 3 months, 10 matched controls were assigned to 6MWT. The primary end point was the change from baseline to week 12 in the distance walked in 6 minutes.The change in mean pulmonary artery pressure and NYHA/WHO functional class and the incidence of clinical worsening were also assessed. Results: There were statistical improvement in the hemodynamics such as slowering of the heart rate( from 113.8±16.0beats/min to 105.0±18.2beats/min,p=0.005),oxygen saturation significantly improved (from 87.7±9.6% to 89.8±9.1% ,p=0.045),the mean pulmonary artery pressure was significantly decreased( from 71.3±24.5mmHg to 59.0±23.4mmHg ,p=0.005),NYHA/WHO functional class was improved as class II-III decreased from 20% to be 8.7% and class III-IV from 26.7% to 4.3%.The 6MWD was significantly slower in the patients compared to the controls as p<0.001 , while it remains nearly the same value after 3 month follow up in the patients. Conclusions: 6MWT is a good diagnostic test for pulmonary arterial hypertension.Sildenafil therapy improves oxygen saturation and decreases the degree of PH.