A case of primary malignant lymphoma involving the hypothalamus is reported. A 31-year-old man was admitted with progressive disorientation. A computed tomography (CT) scan demonstrated a normodense, homogenously enhanced, and nodular shaped mass lesion involving the suprasellar region and extending to the third ventricle. In view of marked cerebral edema, dexamethasone was intravenously administered, at a dose of 4 mg/4 hr, and gradually tapered off. CT scans obtained 10 days after the admission revealed dramatic regression of the lesion, which corresponded with the clinical remission. However, on the 4th day after the discontinuance of corticosteroid, a CT scan demonstrated the recurrence of the mass lesion. Because of the rapid deterioration of consciousness and the progressive enlargement of the mass and dilatation of the ventricles, partial removal of tumor and ventricular drainage were carried out on the 24th day after admission. Dexamethasone was intravenously administered again at a dose of 4 mg/4 hr. A CT scan obtained five days after the operation showed complete disappearance of the lesion, and the patient had became almost alert. CT scans obtained on the 18th day and the 35th day after the operation showed complete disappearance of the mass. However, Magnetic Resonance Imaging revealed a localized area of abnormal signal intensity in the hypothalamus. Diabetes insipidus was continuously observed from the time of admission. A concomitant change of beta-2-microglobulin (beta-2-MG) and immunoglobulin (IgA, IgG, IgM) concentrations in serum and cerebrospinal fluid (CSF) with clinical profiles and CT findings were observed.(ABSTRACT TRUNCATED AT 250 WORDS)