A case of ruptured aneurysm in the hypoplastic proximal anterior cerebral artery (A1 portion) is reported. This 25-years old man complained of the sudden onset of severe headache and vomiting on January 11, 1989. He was referred to our hospital on the same day, and CT scan revealed subarachnoid hemorrhage. Cerebral angiography on the next day revealed an aneurysm in the hypoplastic A1 portion of the right anterior cerebral artery, and no branch was present at the site of the aneurysmal neck. He was operated on using the right pterional approach. The A1 portion was trapped and the aneurysm was removed successfully. The histology of the aneurysm was that of the usual type of the saccular aneurysm. The post-operative course was uneventful. He was discharged with no neurological deficit two months after the operation. As far as we know, there has been no report on a ruptured aneurysm in the hypoplastic A1 portion. We also reviewed the 55 aneurysms in the A1 portion that have been reported in the literature.