A case of incidentally diagnosed fenestration of the supraclinoid internal carotid artery (ICA) with an associated aneurysm is presented. We present appearances on magnetic resonance angiography, which have not been previously described. Careful interrogation of the data in volume-rendered and multiplanar reformats was required to make the diagnosis. The relationship between fenestrations and aneurysms in the anterior circulation is discussed both in general terms and with specific regard to the supraclinoid ICA. We also review the embryology of the distal ICA, which may help explain the adult anatomy of this rare lesion as a failure of caudal separation of the rostral and caudal divisions of the primitive distal ICA.