Objective: We sought to determine the indications and efficacy of oral Sildenafil therapy in patients who underwent congenital heart surgery. Methods: A retrospective review of 77 patients with various congenital heart diseases, who received Sildenafil therapy postoperatively, was performed. Sildenafil citrate (Viagra [Pfizer, Seoul, Korea]) was administered orally 3~4 times daily at a dose of 1 mg/kg per dose. Results: Median age at operation was 19 months (6 days – 17 years). Functionally single ventricle with or without pulmonary stenosis/atresia was the most common diagnosis (40/77, 51.9%). Forty patients had been on nitric oxide (NO) inhalation therapy for 1 – 27 days (median: 2 days) before Sildenafil treatment, and NO inhalation therapy could be discontinued within a median duration of 2 days afterward. With respect to the clinical indications, patients were categorized into 4 groups: 1) to prevent pulmonary hypertensive crisis (PHC) in 28, 2) to lower the pulmonary vascular resistance in patients with bidirectional cavopulmonary shunt or Fontan operation in 34, 3) to stabilize pulmonary vascular reactivity after palliative operations in 8, and 4) to facilitate the maturation of the peripheral pulmonary vascular beds in 7. Mean pulmonary artery pressure (m-PAP) decreased significantly after the initiation of Sildenafil therapy (mean: 41.1 to 35.7 mmHg, p= 0.024) in group 1. In group 2, m-PAP decreased (mean: 16.7 to 15.5 mmHg, p=0.003) and mean oxygen saturation increased significantly (p= 0.013) after the use of Sildenafil. Conclusion: Sildenafil can be used safely in postoperative patients with congenital heart disease with excellent outcomes.