Clinical and pathological studies were made on 5 patients (1 male and 4 females; average age, 58.2 years) with primary malignant lymphoma of the brain. One case had received long-term immunosuppressive therapy for SLE. The most common initial signs and symptoms were non-specific and non-localized. They included headache, disorientation and consciousness disturbance. During the course, the signs and symptoms consisted of consciousness disturbance (5 cases), hemiparesis (4 cases), headache (3 cases), dementia (2 cases), seizures (2 cases), and diplopia (1 case). The tumors on CT scans appeared as slight hyperdense areas in 3 cases, and as isodense areas in 1 case with enhancement following contrast media infusion, which was compatible with previously reported results. However, the other case showed diffuse hypodense areas without enhancement which has rarely been reported. Multiple lesions were found in 4 cases. Three cases underwent cerebral angiography which demonstrated avascular masses. Pathologically, the tumors were located in the cerebral hemispheres in 5 cases, the basal ganglia and thalamus in 3 cases, the brain stem in 2 cases, and the cerebellum in 2 cases. Three cases were classified as of the diffuse, large cell type, 1 case as small cleaved cell and 1 case as immunoblastic. Thus, the clinical diagnosis of primary malignant lymphoma of the brain still remains difficult because the symptoms and CT findings are so varied.