We report a 39-year-old man with intravascular large B-cell lymphoma (IVLBCL) who had been treated as a case with pulmonary arterial hypertension (PAH) for one year. After he became worse, diffuse pulmonary (18)F-fluorodeoxyglucose (FDG) uptake in positron emission tomography (PET) suggested the existence of IVLBCL in the lung showing normal CT images. The diagnosis was confirmed with random transbronchial lung biopsy, and he was then successfully treated. Since IVLBCL presenting PAH has been rare and is difficult to diagnose, early application of FDG-PET may provide early recognition of the disorder, leading to a better outcome.