History A 68-year-old man was admitted to the hospital for work-up because of generalized fatigue, anorexia, chronic diarrhea, and weight loss. Laboratory work-up revealed an erythrocyte sedimentation rate of 58 mm/h (reference range, 3-23 mm/h), a hemoglobin level of 14.1 g/dL (reference range, 13.8-17.5 g/dL), a leukocyte count of 8.1 × 109/L (reference range, [3.4-9.7] × 109/L), a platelet count of 223 × 109/L (reference range, [158-424] × 109/L), an alkaline phosphatase level of 85 U/L (1.42 μkat/L) (normal level, <142 U/L [2.37 μkat/L]), a serum creatinine level of 93 μmol/L (reference range, 79-125 μmol/L), a serum total protein level of 82 g/L (reference range, 66-81 g/L), a serum albumin level of 39.3 g/L (reference range, 40.2-47.6 g/L), an albumin-to-globulin ratio (a test showing relative amounts of major plasma proteins) of 0.92 (reference range, 0.8-2.0), a urine protein level of 15 mg/dL (normal level, <20 mg/dL), a total serum calcium level of 2.46 mmol/L (reference range, 2.14-2.53 mmol/L), and a carcinoembryonic antigen value of 2.69 μg/L (normal value, <3.4 μg/L). Serology findings were negative for celiac disease. Thyroid function was normal, as were 5-hydroxyindoleacetic acid and chromogranin A levels. Initial radiologic examination included chest radiography and plain abdominal erect radiography. Gastrointestinal endoscopy was performed to rule out inflammatory bowel disease or gastrointestinal neoplasm as a cause of chronic diarrhea. Endoscopic mucosal resection of two polyps from the cardia and duodenal bulb was performed during esophagogastroduodenoscopy, but histologic findings at hematoxylin-eosin staining were normal. Colonoscopy revealed diverticulosis involving the entire colon. Serum immunoelectrophoresis showed a monoclonal band, which was confirmed to be immunoglobulin Mλ at immunofixation. After histologic analysis of the bone marrow biopsy specimen, diagnosis of Waldenström macroglobulinemia was established, and computed tomography (CT) of the thorax, abdomen, and pelvis was requested to depict lymphadenopathy and organomegaly. On the basis of CT findings, two more specimens considered highly sensitive for the CT diagnosis were obtained via minimally invasive biopsy, but the results were negative. Magnetic resonance (MR) imaging was performed a year later to control the progression of CT findings.