Cerebral angiography revealed that hypoplasia of the A1 portion of the anterior cerebral artery is more frequent in patients with aneurysms of the anterior communicating artery than in those with aneurysms of other intracranial arteries. As for the hypoplasia of the A1, the angiographic findings corresponded to the surgical findings in 86.9% of all the patients. In patients with anterior communicating artery aneurysms, the incidence of hypoplasia of the A1 portion was about 3 times higher on the right side than on the left side.