A case of agenesis of the left internal carotid artery with an aneurysm of the anterior communicating artery was reported in detail. The patient was a 48-year-old man who had an episode of subarachnoid hemorrhage about 10 years ago. This time left heminumbness and motor weakness of the left lower extremity occured and he was brought our hospital. Right common carotid angiography, left retrograde brachial serial angiography and aortography demonstrated an agenesis of the left internal carotid artery, an aneurysm of the anterior communicating artery, tortuous megadolic hobasilar artery and anomalies on the circle of Willis such as absence of supra optic portion of the right anterior cerebral artery, and large left posterior communicating artery. We confirmed latter findings by operation. After successful neck clipping of the aneurysm, his clinical findings were remarkably improved.