Purpose <p>The purpose of this review is (1) to describe variability of the carotid-anterior cerebral artery (ACA) anastomosis (infraoptic ACA) and (2) to propose additional classification of bilateral type.</p> Methods <p>The author reviewed many articles reporting carotid-ACA anastomosis, including the authors’ own previous reports, and analyzed the pattern of anomalous arteries with the presence or absence of the normal A1 segment, with particular attention to bilateral types. The data base used was PubMed. The search terms used were “carotid-ACA anastomosis” and “infraoptic ACA”.</p> Results <p>There are three major types of unilateral carotid-ACA anastomosis: type 1, bilateral normal A1s are present; type 2, ipsilateral normal A1 is absent but contralateral A1 is present; and type 3, bilateral normal A1s are absent. There was a strong right-sided predominance. The bilateral type is rare, with no significant laterality in size. The author proposed the bilateral type as type 4, with three subtypes: type 4a had bilateral normal A1, type 4b had unilateral A1, and type 4c had no A1. In most cases of the bilateral type, carotid-ACA anastomosis was classified as type 4c.</p> Conclusion <p>Carotid-ACA anastomosis is clinically significant because of its strong association with an aneurysm at the anterior communicating artery complex. And it is also dangerous in surgical operations in the suprasellar region approached via the anterior cranial fossa, such as aneurysm clipping. Because of symmetry, the bilateral type (type 4) may sometimes be overlooked during the interpretation of magnetic resonance angiography and computed tomography angiography.</p>