A 76-year-old woman was admitted because of hypoxemia. She had been complaining of dyspnea for 2 months. Both plain and enhanced CT of the chest showed no abnormality. Her serum LDH level was markedly elevated. Though lung ventilation scintigraphy showed no abnormality, lung perfusion scintigraphy disclosed multiple perfusion defects. A fluorodeoxyglucose (FDG) positron emission tomography scan revealed slight uptake of FDG in bilateral dorsal lung regions. We performed a transbronchial lung biopsy (TBLB) but made no accurate diagnosis. After 2 weeks of TBLB, she developed neurological symptoms, such as convulsion, anarthria, gait disturbance, and disturbance of consciousness. She died 5 months after admission. We later restudied her TBLB specimen and found the proliferation of many CD20-positive atypical cells in capillary vessels of the alveolar septa, which led us to identify this case as an intravascular large B-cell lymphoma (IVLBCL). Although IVLBCL is a rare disease, it is necessary to include IVLBCL in differential diagnosis in cases of hypoxemia with normal chest CT findings and elevated serum LDH.