•The purpose of this study is to provide an analysis of iliosacral screw insertion sites on the posterior ilium.•Improved outcomes of LC II injuries treated percutaneously marks the importance of a preoperative plan to determine amenability of percutaneous iliosacral screw fixation.•Mapping iliosacral screw start sites can be helpful for preoperative planning of percutaneous pelvic ring injury fixation.•This has demonstrated a relatively predictable region of screw start site on the posterior ilium, which is dependent on the sacral morphology and sacral level.•Iliosacral screws start sites on the posterior ilium have reliable regions that can be used to plan posterior fixation of pelvic ring injuries. Lateral compression type II pelvic ring injuries can be treated with fixation through open or percutaneous approaches depending on the injury pattern and available osseous fixation pathways. The start site of iliosacral screws to stabilize these injuries should be on the unstable posterior iliac fragment; however, our understanding of start sites for iliosacral screws has not been developed. The purpose of this study is to provide an analysis of iliosacral screw start sites on the posterior ilium to help guide treatment of pelvic ring injuries. One-hundred and seventeen consecutive patients at an academic level I trauma center with pelvic ring injuries who underwent surgical treatment with iliosacral screws were included in the final analysis. The start sites of iliosacral screws with confirmed intraosseous placement on a postoperative computed tomography were mapped on the posterior ilium and analyzed according to the sacral segment and type of iliosacral screw. One-hundred and seventeen patients were included in the final analysis. Of the total of 272 iliosacral screw insertion sites analyzed, 145 (53%) were sacroiliac-style screws and 127 (47%) were transsacral screws. The insertion sites for sacroiliac-style screws and transsacral screws at different sacral segment levels can vary but have predictable regions on the posterior ilium relative to reliable osseous landmarks. Iliosacral screws start sites on the posterior ilium have reliable regions that can be used to plan posterior fixation of pelvic ring injuries.