We report a 45-year-old man with monocytosis and right hemiparesis. The patient suffered from an acute myocardial infarction from which he recovered completely when he was 42 years old. One year prior to his death, he was found to have increase in monocyte count (35.5% of leukocytes) in peripheral blood and splenomegaly; he was admitted to the hematology service of our hospital. He was diagnosed as having chronic myelomonocytic leukemia after bone marrow examination. He was treated with radiation therapy with improvement in splenomegaly. In May of 1995, he had fever, anemia, and thrombocytopenia for which he needed daily blood transfusion. In November of 1995, he had an onset of weakness in his right hand, and neurologic consultation was asked for in November 27, 1995. Neurologic examination revealed a chronically ill japanese man in no acute distress. He was alert and not demented. Higher cerebral functions were intact. Cranial nerve examination revealed right facial paresis of the central type. Motor-wise, he was right hemiparetic. Generalized muscle wasting was noted apparently due to the chronic debilitating disease. Deep tendon reflexes were within normal range in the right upper extremity, but were diminished in other areas. Sensation was intact, and no meningeal signs were noted. Pertinent laboratory findings were as follows: Hb 8 g/dl, RBC 238 x 10(4)/microliter, WBC 2,900/microliter (band 1.0%, seg 18.5%, lym 28.0%, mono 44.0%, Baso 2.5%), Plt 13 x 10(4)/microliter, PT 16.6"/10.9", APTT 44.7"/35.0". CSF contained 87 mg/dl of protein, 155 mg/dl of glucose and 2 mononuclear cells/microliter. Bone marrow was slightly hypercellular with mild increase in blast forms. No chromosome abnormality was found. CT and MRI revealed a large mass in the left fronto-parietal region and the meninges showed marked thickening with enhancement after gadolinium-DTPA in MRI. The patient was treated with glycerol and steroid, but the subsequent course was complicated by a seizure, agitation, and pneumonia. He died from respiratory failure on January 13, 1996. The patient was discussed in a neurologic CPC and the chief discussant arrived at the conclusion that the patient had chronic myelomonocytic leukemia with infiltration of leukemic cells into meninges and the parenchyme of the cerebrum. Thickening of the dura was thought to be in part a reaction to the subdural hematoma as well as to leukemic cells along the meninges. Postmortem examination revealed hypercellular bone marrow with increase in monocytic cells (more than 20%). The lungs showed pneumonia with scattered old tuberculous lesions. The heart showed an old myocardial infarction in the posterior wall of the left ventricle. The brain showed an old chronic subdural hematoma in the left fronto-temporal region and a cystic mass lesion in the left frontoparietal region. The mass was hypercellular and most of them were monocytes. The dura mater showed reactive thickening without leukemic cell infiltration. It was concluded that this patient had chronic myelomonocytic leukemia with a formation of leukemic mass in the brain. Pathologists thought that the mass was a hematogenous spread. It is rare for chronic myelomonocytic leukemia to form a mass lesion in the brain.