Background: Chest computed tomography (CT) is generally considered as the most useful examination to research the cause of hypoxia. However some diseases of the lung are not detected by CT.Case 1: Seventy-four years old male having past history of cerebral infarction went to home doctor because of dyspnea. Chest X-ray was normal although oximeter showed low SpO2 (89%). He was admitted to our hospital for further examination. Although hypoxia progressed, chest CT showed no abnormal shadow compatible with low SpO2. Gallium-67 (G67) scintigraphy showed abnormal diffuse uptake in all over the lung field. Soluble interleukin 2 receptor was 400 IU/ml. Skin biopsy was performed and lymphoma cells were detected intravascular lesion. We diagnosed him as intravascular lymphoma (large B cell lymphoma). Chemotherapy with RCHOP was performed and symptoms were improved.Case 2: Sixty-nine male was admitted to our hospital because of dyspnea worsening. Low oxygen partial pressure (PaO2= 47.3 mmHg) and low grade fever (37.5 degree) was observed. Chest CT showed slight emphysema and nodular opacity in apex of the lung. Hypoxia kept on worsening after hospitalization. Cause of hypoxia was not detected from chest CT. Seven days after hospitalization, disseminated intravascular coagulation (DIC) developed. High value with soluble interleukin 2 receptor was observed (sIL2R= 6450 U/ml). Myelogram showed no atypical lymphocyte. Two weeks after hospitalization he died and the autopsy was performed. Perivascular granulomatous lymphoma cells proliferations were detected in all over the lung and other multiple organs (skin, liver, spleen, kidney, thyroid, bone marrow, esophagus, stomach, prostate, heart, and lymph node). We diagnosed him as angiocentric B cell lymphoma. Conclusion, Large B cell lymphoma should be considered as one of the differential diagnosis in hypoxia patients lacking explainable abnormal shadow on chest CT. Background: Chest computed tomography (CT) is generally considered as the most useful examination to research the cause of hypoxia. However some diseases of the lung are not detected by CT. Case 1: Seventy-four years old male having past history of cerebral infarction went to home doctor because of dyspnea. Chest X-ray was normal although oximeter showed low SpO2 (89%). He was admitted to our hospital for further examination. Although hypoxia progressed, chest CT showed no abnormal shadow compatible with low SpO2. Gallium-67 (G67) scintigraphy showed abnormal diffuse uptake in all over the lung field. Soluble interleukin 2 receptor was 400 IU/ml. Skin biopsy was performed and lymphoma cells were detected intravascular lesion. We diagnosed him as intravascular lymphoma (large B cell lymphoma). Chemotherapy with RCHOP was performed and symptoms were improved. Case 2: Sixty-nine male was admitted to our hospital because of dyspnea worsening. Low oxygen partial pressure (PaO2= 47.3 mmHg) and low grade fever (37.5 degree) was observed. Chest CT showed slight emphysema and nodular opacity in apex of the lung. Hypoxia kept on worsening after hospitalization. Cause of hypoxia was not detected from chest CT. Seven days after hospitalization, disseminated intravascular coagulation (DIC) developed. High value with soluble interleukin 2 receptor was observed (sIL2R= 6450 U/ml). Myelogram showed no atypical lymphocyte. Two weeks after hospitalization he died and the autopsy was performed. Perivascular granulomatous lymphoma cells proliferations were detected in all over the lung and other multiple organs (skin, liver, spleen, kidney, thyroid, bone marrow, esophagus, stomach, prostate, heart, and lymph node). We diagnosed him as angiocentric B cell lymphoma. Conclusion, Large B cell lymphoma should be considered as one of the differential diagnosis in hypoxia patients lacking explainable abnormal shadow on chest CT.