Background: The recent development of various effective drugs, such as epoprostenol, sildenafil, and bosentan, has improved the prognosis for patients with idiopathic pulmonary arterial hypertension (IPAH). This study sought to determine survival rates and to identify predictive prognostic factors in patients with IPAH in the current era of combination therapy with new and more effective vasodilators. Methods and Results: In 65 consecutive IPAH patients treated from 2004 to 2009, hemodynamic parameters were significantly improved and brain natriuretic peptide was significantly decreased by combination therapy (observation period: 35±18 months). The Kaplan-Meier survival curves were determined, and 22 prognostic variables, including 9 hemodynamic variables and 6 biomarkers, were evaluated to obtain the best variables. The 1-year and 3-year survival rates were 98% and 86%, respectively. Only the platelet level was correlated with death (P<0.05), and the platelet level was significantly correlated with mean pulmonary arterial pressure (P<0.01). Patients with a lower platelet level (<20×10⁴/μl (median value)) before treatment had a higher mortality rate compared to the other patients (78% vs. 95% for 3-year survival, P<0.01). Conclusions: Combination therapy contributed to an improvement in the prognosis of IPAH patients. Platelet level is a significant prognostic predictor in this new treatment era. (Circ J 2012; 76: 1494-1500)