Arteriovenous malformation and multiple anomalies of the circle of Willis.

A case is presented of subarachnoid hemorrhage from an arteriovenous malformation (AVM) involving the left middle cerebral artery in the circle of Willis, seen in association with multiple anomalies of the circle. This is an extremely unusual location for such a malformation. The possible etiology of the AVM and its relationship to the associated anomalies is discussed.