A 70-year-old woman who had been treated for bipolar disorder and dementia was admitted to the intensive care unit of a university hospital with severe dyspnea; pulmonary arterial hypertension was diagnosed after cardiac catheterization was performed. Computed tomography pulmonary angiography showed typical signs of chronic thrombosis in the proximal pulmonary artery without an adequate amount of fresh thrombi, which appeared to be the cause of the elevation in pulmonary artery pressure, and resulted in severe hypoxemia. Therefore, the pulmonary arterial hypertension was classified as belonging to the chronic thromboembolic pulmonary hypertension subgroup. Although the patient's respiratory condition was classified as World Health Organization class IV, she was treated with the combination of oral ambrisentan and tadalafil, rather than intravenous epoprostenol, which she was unable to tolerate. Consequently, both her symptom and hemodynamic status showed rapid improvement with only oral pulmonary vasodilators. This case demonstrates the efficacy of oral treatment alone in elderly patients with severe chronic thromboembolic pulmonary hypertension.