Two cases of traumatic internal carotid artery occlusion probably related to the seat belt shoulder strap are reported. Case 1. A 20-year-old woman was driving and was struck on the right front side of her car by another car. There were neither bruises, abrasions on her neck, nor weakness in her extremities. About 4 hours later, she developed left hemiplegia, and CT scan taken on the following day revealed low density areas in the capsulostriatal area on the right. The right carotid angiography revealed occlusion of the internal carotid artery about 3 cm distal to the bifurcation. Case 2. A 43-year-old man was driving and was struck on the front of his car by a hard iron railing. He sustained a sternum fracture, but there was no disturbance of consciousness or paresis of the extremities. His neck was unremarkable externally. About 50 days later, he developed left hemiplegia. CT scan and MRI revealed a massive infarction in the distribution of the right middle cerebral artery territories. The carotid angiography revealed occlusion of the right internal carotid artery about 3 cm distal to the bifurcation. In each cases, the driver was wearing a three-point shoulder seatbelt when the car was struck on the front or on the right front. Previous experimental studies have revealed in these situations the neck is flexed right anteriorly, and then quickly overextended left posteriorly. The overextension of the neck probably injured the intima of the internal carotid artery ipsilateral to the shoulder fixed in the seatbelt, resulting in the subsequent occlusion by a thrombus.