A 58-year-old woman was admitted with refractory fever despite receiving broad-spectrum antibiotics. She had hypoxemia, severe anemia, elevated levels of serum lactic dehydrogenase and soluble interleukin-2 receptor, and a positive direct Coombs test, which suggested an underlying autoimmune hemolytic anemia (AIHA). Chest computed tomography (CT) showed no abnormal findings, but she had hypoxia, and her alveolar-arterial oxygen difference (A-aDO2) was increased. A random transbronchial lung biopsy (TBLB) was performed, and pathological analysis showed massive proliferation of tumor cells in the lumina of the small vessels. Intravascular large B-cell lymphoma (IVLBCL) was diagnosed, and her general status improved after chemotherapy.