A case of arterio-venous malformation (AVM) at the right parietal area in association with agenesis of the left internal carotid artery (ICA) is described. A 27-year-old male complained of severe headache and vomiting on September 27th in 1980. He lost his consciousness and became stuporous within several minutes after the ictus. He was then transferred to Kurume University Hospital. On admission, he was semicomatose, anisocoria and left hemiparesis marked in the lower limbs, and bilateral Babinski reflex were noted. A emergency CT scan revealed a round high density area suggesting intracerebral hematoma at the right parieto-occipital area. Right retrograde brachial angiography was then performed which showed a small AVM at the right parietal lobe mainly fed by the right posterior parietal arterial branch, and drained into the superior sagittal sinus, via the subcortial veins. Left MCA bilateral PCA and SCA also were demonstrated on the angiogram. The angiogram suggested on abnormality of the Willis ring. Emergency operation fro the small AVM associated with intracerebral hematoma was then performed. The AVM was sufficiently removed and approximately 80 gr. of intracerebral clots were also evacuated completely. Histological examination indicated a typical small AVM. The postoperative course was uneventful. Left CAG performed by seldinger method from the femoral artery after the operation showed no demonstration of the left ICA. the left common carotid artery was terminated as the external carotid artery without carotid bifurcation at the neck. The left ophthalmic artery was fed by the meningeal artery of the internal maxillary artery. Left vertebrobasilar system was normal. From an angiogram, the agenesis of the left ICA was most suspected. An agenesis of ICA was uncommon among the literature in which there was no case with this anomaly associated with cerebral AVM. The embryological consideration about this case was mainly discussed.