STUDY DESIGN: Anatomic study of the internal carotid artery (ICA) location with respect to C1 based on computed tomography (CT) scans with contrast medium. OBJECTIVE: To measure the location of the ICA relative to the anterior aspect of C1 to assess the risk of placing C1-C2 transarticular or C1 lateral mass screws. SUMMARY OF BACKGROUND DATA: Vertebral artery injury is a known risk from placement of screws in C1. A previous case report revealed an ideally placed C1-C2 transarticular screw abutting and narrowing the ICA. The risk of ICA injury from C1 screws is unknown. METHODS: Fifty random head and neck CT scans with contrast medium were retrospectively analyzed. Measurements were taken bilaterally including the closest distance from the ICA lumen to C1 and the distance from the medial edge of the ICA to a line drawn along the medial border of the foramen transversarium. The risk of inserting bicortical C1-C2 transarticular and C1 lateral mass screws was estimated based on these measurements. RESULTS: The mean distance from the ICA to C1 was 2.88 mm on the left and 2.89 mm on the right. The ICA lumen was medial to the foramen transversarium in 42 (84%) of 50 cases (mean: 2.78 mm on the left and 3.00 mm on the right). The proximity of the ICA to C1 posed moderate risk in 46% of cases and high risk in 12% (on at least one side). CONCLUSION: Because of the risk of ICA injury from a drill bit or the tip of a bicortical screw, we recommend preoperative CT scan with contrast medium in all cases in which a screw is to be placed into C1. If the ICA is in close proximity to the anterior border of C1, unicortical fixation or a different fusion technique should be considered.