A 71-year-old man developed intermittent fever with high serum lactate dehydrogenase in December 2006. Although persistent high grade fever and hypoxia appeared in July 2007, chest and abdominal CT showed no abnormal findings. On admission to our hospital in September 2007, chest CT demonstrated multiple micro-nodules in both lungs. Since diffuse lung capacity was extremely low and lung perfusion decreased diffusely, we performed video-associated lung biopsy and intravascular lymphoma was diagnosed. Histological examination of the lung specimen revealed a small number of atypical lymphocytes and some thrombi in capillaries. These findings suggested that severe hypoxia was caused mainly by diffuse pulmonary vasoconstriction due to some small thrombi and tumor cells, not by their embolization. Hypoxia improved dramatically in a few days after initiation of chemotherapy including rituximab.