In 1987, a 69-year-old female suffering from epilepsy and right hemiparesis was admitted to the hospital and given conservative therapy. During the next three weeks she became a stuporous state. Steroids were then prescribed, and she made a complete recovery. Following hormonal and immunological investigations, a diagnosis of Hashimoto's disease with hypothyroidism was made. Following her discharge, thyroxin and anticonvulsant treatments continued. On May 30, 1989, at the age of seventy-one, the patient was found lying unconscious at home, and taken to our hospital. She had signs of a confused state, and seizures of her right arm on admission. Cranial computed tomography (CT) revealed a low density area in the left parieto-occipital lobe, which was heterogeneously enhanced by contrast medium. Magnetic resonance imaging (MRI) showed a lesion of low signal intensity in T1-weighted images and high signal intensity in T2-weighted images, which was homogeneously enhanced by Gd DTPA. Cerebral angiography disclosed no abnormality. Two weeks later, follow-up CT showed that the lesion had become larger and markedly enhanced. A brain tumor, probably malignant lymphoma, was suspected. On June 14, stereotaxic biopsy of the enhanced lesion was performed. Histopathological examination disclosed dense infiltration of the entire walls of many small parenchymal vessels, both arterioles and venules, by lymphocytes. Treatment for angiitis (betamethasone 16 mg daily) was started on June 20, then gradually tapered, and the lesion diminished on CT. Our case has some features of both isolated angiitis of the central nervous system and encephalopathy in compensated Hashimoto's disease. It is suggested that some common base of the autoimmune diseases exists in this case.