The placement of iliosacral screws for the stabilization of pelvic ring lesions is technically demanding. The postoperative computed tomography scans of 31 patients who had 57 iliosacral screws placed for various indications were studied to determine the proximity of these screws to neurovascular structures. The closest distance of the screws from the S1 foramen averaged 3 mm. (range, 0-10.5 mm); the average closest distance to the anterior cortex of the sacral ala was 4.8 mm (range, 0-15.3 mm). The corridor for the insertion of the screws between the S1 foramen and the anterior cortex of the sacrum averaged 21.7 mm (range, 16.2-28.9 mm). Trigonometric analysis of these dimensions suggests that deviations of the surgeon's hand by as little as 4° may direct iliosacral screws either into the S1 foramina or through the anterior cortex of the sacrum.