A 30-year-old man with New York Heart Association functional class IV was hospitalized with chief complaints of syncope and dyspnea, which had begun in November 1998. He had no medical or family history of note. He had smoked one pack of cigarettes per day for 10 years. He was noted to have jugular vein dilatation, a loud pulmonary component of S2, an S3 gallop, right ventricular lift, and hepatomegaly. Arterial blood gas analysis (room air) revealed a pH of 7.45, Po2 of 66.0 mm Hg, and Pco2 of 26.7 mm Hg. The ECG showed right ventricular hypertrophy. The chest radiograph revealed prominent pulmonary arteries and interstitial shadows in the right lower lung field, indicating pulmonary congestion. The results of a workup for secondary causes of pulmonary hypertension were normal. Pulmonary function tests showed reduced diffusing capacity of the lung for carbon monoxide (40.0% of predicted). Ventilation-perfusion images revealed normal ventilation and diffuse focal areas of hypoperfusion. Right heart catheterization revealed severe pulmonary arterial hypertension (mean pulmonary arterial pressure, 70 mm Hg; pulmonary vascular resistance, 15.1 Wood U; mean right atrial pressure, 17 mm Hg; pulmonary capillary wedge pressure, 10 mm Hg; cardiac output, 3.97 L/min). Ischemic heart disease, valvular disease, and cardiomyopathy were excluded by echocardiography and cardiac catheterization. Based on these findings and the presence of pulmonary congestion, PVOD, a subtype of PPH, was strongly suspected. Continuous IV PGI2 was immediately begun at 1 ng/kg/min and was progressively titrated by 10 ng/kg/min in a week, because the patient’s general condition deteriorated critically. Thereafter, the PGI2 dosage was gradually increased by 1 ng/kg/min per week. Two episodes of pulmonary edema occurred during follow-up. However, the patient’s symptoms and chest radiographic findings were significantly improved when the dose of PGI2 was titrated to 29 ng/kg/min. Furthermore, his 6-min walk distance increased from 90 to 450 m, and his mean pulmonary artery pressure decreased from 70 to 36 mm Hg. He was discharged from the hospital about 4 months after the start of PGI2 administration.