The case of a 34-year-old Caucasian male with subarachnoid hemorrahge is reported. The right carotid arteriogram shows the typical internal carotid artery occlusion at the syphon, with an abnormal vascular network at the base of the brain; the right anterior and middle cerebral arteries fill from the vascular network and through meningocortical anastomoses; the right posterior communicating and cerebral arteries are tortuous and hypertrophic. The left carotid arteriogram shows hypertrophy of the common and internal carotid arteries; both anterior and middle cerebral arteries fill from this side; the anterior communicating artery is thick (foetal type) and a median anterior cerebral artery to the corpus callosum is present. It is the authors' view that the "moyamoya" disease is a congenital one, since the abnormal vascular network at the base of the brain would not be necessary, in this case, to preserve good collateral circulation to the right cerebral hemisphere. No other case with the typical findings of "moyamoya" disease on one side and hypertrophy of the carotid arteries on the other was found in the literature.