Angiographic and operative investigations revealed an anomalous branch of the internal carotid artery (ICA) in a patient with an anterior communicating artery (ACoA) aneurysm. The anomalous vessel originated from the right ICA at the level of the ophthalmic artery, and pursued an infraoptic and prechiasmatic path to supply both pericallosal arteries. The clinical features and possible genesis of this anomaly are discussed. This irregularity is frequently associated with intracranial aneurysms, especially those of the ACoA, and with other anomalies.