In Japan, a large number of surgical treatments for unruptured intracranial aneurysms have been performed. On the other hands, it is known that surgical treatment for unruptured intracranial aneurysms is the most frequent source of lawsuits in the neurosurgical field. Neurosurgeons have the duty to disclose all risks and consequences of a proposed surgical procedure to each patient. Moreover, precise knowledge of surgical risks would be helpful in preventing complications from occurring. However, it is almost impossible that a single surgeon or an institute have experiences with all possible surgical complications because there is limitation of the number of surgery. In this review series, we attempted an exhaustive bibliographic survey of the possible surgical complications including exceptional ones, and then, determined the frequency of each complications as far as possible quantitatively. In this paper, we focused on the complications of surgical treatment for anterior cerebral artery aneurysms.