We report a 65-year-old man with progressive loss of vision and consciousness disturbance. The patient was well until his age of 63 when he was found to have a gastric cancer. He was treated by the tumor resection and chemotherapy; he was apparently well, but hepatic metastases were found in the next year (1996). In June, 1996, he noted an onset of blurred vision more on the left. He was admitted to the ophthalmology service of our hospital on July 14, 1996. His vision was 0.8 on the right and 0.15 on the left. He was treated with oral prednisolone with slight improvement. He was also found to have IgM kappa-type monoclonal gammopathy; Bence-Jones protein was positive and a bone marrow aspiration revealed that approximately 10% of bone marrow cells were atypical plasma cells. His vision had progressively got worse and he became blind by the end of October 1996. A chest X-ray and cranial CT scan revealed multiple abnormal nodular densities. In the middle of November 1996, he became confused, disoriented and agitated. His mental symptoms had progressively became worse, and a neurologic consultation was asked on December 10, 1996. Neurologic examination revealed that he was somnolent with decreased attention to his surroundings. He showed marked disorientation and memory loss. Higher cerebral functions appeared intact. He was able to recognize only light and dark. Pupils were moderately dilated with very sluggish light reflex remained. Vertical gaze was moderately restricted and horizontal nystagmus was noted upon left and right lateral gaze. The remaining of the neurologic examination were unremarkable. General physical examination revealed hepatosplenomegaly; the liver was palpable by 3 cm below the right costal margin. Laboratory examination revealed anemia (Hb10.1 g/dl) and thrombocytopenia (43,000/microliter). A cranial CT scan and MRI revealed a mass lesion in involving the chiasmatic and bilateral hypothalamic areas. The tumor showed intense homogeneous enhancement after Gd-DTPA infusion. The patient was treated with dexamethasone and radiation. After 9 Gray radiation, he showed deterioration in the sensorium; a cranial CT scan revealed a hydrocephalus of the right ventricle with the midline shift towards left. The radiation was discontinued. The subsequent clinical course was complicated by aspiration pneumonia and thrombocytopenia. He expired on January 4, 1997. The patient was discussed in a neurological CPC and the chief discussant arrived at the conclusion that the patient had systemic malignant lymphoma with metastasis to the brain judging from the characteristics of MRI and CT findings. Opinions were divided between malignant lymphoma and metastatic brain tumor. Post-mortem examination revealed plasmacytoid lymphocytic infiltration in the bone marrow. Immunologically, these cells were positive for IgM and kappa-type light chain. These plasmacytic infiltrations were seen in the lungs and lymph nodes. These findings were consistent with the diagnosis of Waldenström's macroglobulinemia. In the liver metastatic cancer tissues were seen; microscopic pictures were essentially similar to those of resected gastric cancer. No local recidive was noted in the stomach. In the central nervous system, a necrotic tissue was involving the hypothalamic area bilaterally; no clear neoplastic cells were found, however, lymphocytic and plasmacytic infiltrations were seen in the perivascular space. In the optic nerves, loss of myelin and axons were seen. These findings most likely mass formation from macroglobulinemia which underwent necrotic change after radiation. Mass formation in the brain is rare for Waldenström's macroglobulinemia, although it has been reported. The relation between gastric cancer and macroglobulinemia in this patient is unclear.