Background and objectives: The aim of this study was to investigate clinical characteristics and prognostic factors in Korean patients with severe pulmonary hypertension (PHT). Methods: From 2000 to 2005, a total of 169 patients with severe PHT were enrolled and divided into two groups; group I (the dead group, 34 patients, 60.0짹15.7 years old, 18 males), group II (the survived group, 135 patients, 57.2짹16.9 years old, 65 males). Clinical, electrocardiographic, and echocardiographic variables were analyzed between the groups. Results The most frequent cause of severe PHT was valvular heart disease (VHD) (53 patients: 31.4%). Other causes were as follows; pulmonary thromboembolism (29 patients: 17.2%), ischemic cardiomyopathy (17 patients: 10.1%), cor pulmonale (15 patients: 8.9%), congenital heart defect (9 patients: 5.3%), idiopathic dilated cardiomyopathy (8 patients: 4.7%), collagen vascular disease (8 patients: 4.7%), miscellaneous (30 patients: 17.7%). Common presenting clinical symptoms were dyspnea on exertion, leg edema, cough, chest discomfort. NYHA functional class at presentation was significantly higher in group I than in group II (3.7짹0.5 in group I vs 3.1짹0.8 in group II, p<0.001). Atrial fibrillation (AF) was significantly prevalent in group I than in group II (47.6% in group I vs 21.8% in group II, p=0.013). Left ventricular ejection fraction at presentation was significantly lower in group I than in group II (47.3짹15.2% in group I vs 57.1짹15.7% in group II, p=0.002). Mortality rate was significantly lower in patients who improved NYHA functional class (p=0.001). Conclusions: Our study demonstrated that VHD was the most common cause of severe PHT in Korean patients. NYHA functional class, AF, and LVEF at presentation were significant predictors of prognosis in patients with severe PHT.