Among the anomalies of internal carotid artery, agenesis and aplasia recur with a low rate: only about sixty cases are reported in literature. We observed a case of aplasia of the left internal carotid in a 49 year old male suffering from hypertension and showing cerebro-vascular symptoms due to TSA pathology: it was studied with arterial angiography and duplex scanner. The angiographic examination raised a suspicion of left internal carotid thrombosis; on the contrary the duplex scanner revealed a correct diagnosis of carotid aplasia.