A case of agenesis of the left internal carotid artery, common carotid artery, and the main trunk of the external carotid artery with multiple cerebral aneurysms is presented. This case was diagnosed by angiography and computed tomography scanning and confirmed by operation. Correlation between the anomaly of the circle of Willis based on the absence of the internal carotid artery and the development of cerebral aneurysm is discussed on the basis of the reported cases.