It is shown that phosphodiesterase type 5 (PDE5) inhibitors such as sildenafil can modulate pulmonary arterial hypertension (PAH) via increasing the level of guanosine-3,5-cyclic monophosphate (cGMP) and decreases pulmonary artery pressure (PAP). In this study we determined the effectiveness of sildenafil and compared its efficacy with inhaled nasal oxygen (O₂) during cardiac catheterization in patients with congenital heart diseases (CHD) and PAH, as a test of feasibility for surgical repair of the patients. We studied 15 patients, 9 male and 6 female, with a mean age of 8.3 years. Hemodynamic measurements were made at baseline, after O₂ administration for 20 min (5 L/min by mask), and then 45 min after administration of a single dose of sildenafil (0.5 mg/kg orally or via nasogastric tube). Mean PAP at baseline was 72.2 ± 12.54 mm Hg and was reduced by sildenafil to 52.5 ± 9.6 and by O₂ to 61.3 ± 10.39. Both sildenafil and O₂ decreased PAP effectively (p = 0.08 and p = 0.04, respectively). Pulmonary vascular resistance (PVR) was calculated for 12 patients, with a baseline level of 9.08 ± 1.09 mm Hg · L⁻¹ · min, which was significantly decreased by O₂, to 3.74 ± 0.43, and by sildenafil, to 5.93 ± 0.75 (p = 0.005 and p = 0.05, respectively). Sildenafil, as a single oral dose, can effectively reduce PAP and PVR. This novel PDE5 inhibitor can be used for assessment of feasibility of operation for patients with CHD and PAH when inhaled NO is not available.