Objectives: To study the short term effects of sildenafil citrate therapy in patients with secondary pulmonary hypertension. Methods: Forty patients with known symptomatic secondary pulmonary hypertension due to valvular heart disease, chronic thromboembolic disease, chronic obstructive pulmonary disease, interstitial pulmonary fibrosis, and idiopathic dilated cardiomyopathy were included in this phase II study. Patients were allocated in a randomized, placebo controlled design to either sildenafil or placebo for 6 weeks. Baseline and 6 week follow up included assessment of hemodynamic parameters, functional class using the NYHA classification, echocardiographic measurements of pulmonary artery systolic pressure and left ventricular ejection fraction. Results: The mean NYHA class at 6 weeks was 2.05 ± 0.4 in the sildenafil group versus 2.6± 0.6 in the placebo group, p= 0.02. The mean systolic pulmonary artery pressure significantly decreased in the sildenafil group at 6 weeks (43 ± 4 mmHg), compared to placebo patients (53 ± 7 mmHg), p= 0.02. Ejection fraction was higher in the sildenafil group, 59± 12% versus 54± 14% in the placebo group, but did not reach statistically significant difference. Sildenafil was well tolerated with minimal side effects. Conclusion: Our data suggest that sildenafil therapy may provide benefits to selected patients with pulmonary hypertension secondary to cardiac or pulmonary diseases.