Six autopsy cases of subcortical hematoma caused by CAA were examined to elucidate the primary site of hemorrhage. Immunohistochemistry for amyloid beta-protein (A beta) revealed extensive CAA in the intrasulcal meningeal vessels rather than in the cerebral cortical vessels. All of the examined cases had multiple hematomas in the subarachnoid space, mainly in the cerebral sulci, as well as intracerebral hematomas. Each intracerebral hematoma was connected to the subarachnoid hematomas at the depth of cerebral sulci or through the lateral side of the cortex. There was no debris of the cerebral cortical tissue in the subarachnoid hematomas. In case 2, another solitary subarachnoid hematoma, which was not connected to any intracerebral hematoma, was seen. In all of these subarachnoid hematomas, many ruptured A beta-immunopositive arteries were observed. These ruptured arteries did not accompany any debris of the brain tissue, some of them were large in diameter (250-300 microm), and several of them were far from the cerebral cortex. Therefore, it was considered that they were not cortical arteries but meningeal arteries. Within the cerebral cortex, there were only a few ruptured arteries associated with small hemorrhages. There were no ruptured vessels within the intracerebral hematomas. There was a strong suggestion that all of the subarachnoid hematomas, including the solitary one in case 2, originated from the rupture of the meningeal arteries. The present study indicates that in some cases of subcortical hematoma caused by CAA, the primary hemorrhage occurs in the subarachnoid space, in particular the cerebral sulci, because of rupture of multiple meningeal arteries. Infarction occurs subsequently in the cortex around the hematoma, the hematoma penetrates into the brain parenchyma, and finally, a subcortical hematoma is formed.