Cerebral vascular lesions of 26 cases in systemic lupus erythematosus during a period from 1963 to 1978 were examined histologically and the following conclusions were made: 1. The prominent vascular changes of the brain were thrombosis, fibrinoid degeneration, endothelial swelling and proliferation, arteriolosclerosis, and perivascular infiltration of inflammatory cells. 2. From clinico-pathological viewpoints, thrombosis seemed to play an important role in the development of neurological signs. In five cases, characteristic granular or homogeneous thrombi were observed in the small blood vessels including venule. Infarct without proved vascular obstruction but probably due to thrombosis was seen in four cases. The true character of the granular thrombi was not determined, either electronmicroscopically or immunohistochemically. These suggested the presence of a tendency for in situ formation of thrombus. 3. Fibrinoid degeneration seen in four cases mainly affected arterile of less than 50 micrometer in diameter in the cerebral cortex, basal ganglia, and brain stem. This change of arteriole did not play a significant role in neurological signs. 4. Endothelial swelling and proliferation of the small blood vessels were prominent in the cases with thrombosis and fibrinoid degeneration. 5. Perivascular infiltration of the inflammatory cells was observed in about one-half of the cases but its significance was not clear.