Posterior circulation aneurysms, especially aneurysms of the posterior cerebral artery-posterior communicating artery junction (PA-Pcom AN), with agenesis or occlusion of the internal carotid artery (ICA) are rare. We experienced two such cases. One patient had lt. PC-Pcom AN with ipsilateral ICA occlusion, the other blt. PC-Pcom ANs with blt. ICAs occlusion.<BR>Case 1, a 27-year-old man who had been treated for homocystinuria, suffered from subarachnoid hemorrhage (SAH) on January 26, 1980. He was admitted with no neurological deficit three days after the SAH. Carotid angiography revealed complete occlusion of the lt. ICA at its origin. A saccular aneurysm was recognized at the junction of PCA and dilated PcomA. Thirty-one days after the onset the aneurysm was clipped, but he showed disturbance of consciousness and rt. hemiplegia after the operation. He died of sepsis fifty-one days later. Postoperative angiography had shown disappearance of the aneurysm and poor filling of the PcomA.<BR>Case 2, a 52-year-old man suffered from SAH on March 25, 1986 and the next day he was admitted in a drowsy state with signs of meningeal irritation. Blt. ICAs were not opacified intracranially by carotid angiography. Anterior and middle cerebral arteries of both sides were supplied with vertebro-basilar system and blt. PC-Pcom ANs were recognized. Thirty-five days after admission clipping of the responsible lt. PC-Pcom AN and lt. STA-MCA anastomosis were performed. He had temporary lt. hemiparesis but was discharged twenty-three days after the operation without neurological deficits. Postoperative angiography showed disappearance of the aneurysm and good patency of the anastomosis, but the lt. PcomA was not opacified.<BR>We suppose the change of hemodynamics by the occlusion of ICA as the cause of the formation and growth of such aneurysm.<BR>During the operation, the dilated and tortuous PcomA disturbed the procedures and both patients showed neurological deficits inspite of an uneventful operation. In such cases, therefore, considering the difficulty of the operative procedures and the subtle balance of hemodynamics, we should perform the operation more carefully. We think the STA-MCA anastomosis is a good means of preventing complications in these operations.