A 45-year-old man was admitted due to acute pancreatitis. A severe lactic acidosis was found. Following active therapy, the signs and symptoms from pancreatitis was improved, but acidosis was exacerbated. FDG PET/CT images were acquired to investigate the etiology of lactic acidosis and/or other unknown pathology. The images showed widespread abnormal FDG activity in the bone marrows throughout the body, suggestive of hematologic malignancy, which was confirmed as primary osseous non-Hodgkin lymphoma following a histopathological examination of the bone marrow. Chemotherapy against lymphoma was initiated and status of the lactic acidosis was rapidly corrected.