Nineteen cases of external rotation (open book) injury of the pelvis were studies retrospectively. An apparent vertical displacement of the hemipelvis was detected on anteroposterior radiographic views in association with the separation of the symphysis pubis and opening of the sacroiliac joint. This could be confused with a vertically unstable situation; however, careful examination of the radiographs revealed that the public bone on the side of injury was displaced inferiorly. The articular portion of the corresponding sacroiliac joint on computed tomography was opened anteriorly, and the hemipelvis on the same side was externally rotated in all cases (indicating that the posterior sacroiliac ligaments were intact). An anatomic study was then performed on 10 cadaveric pelves. The symphysis pubis and the anterior sacroiliac ligament on one side were sharply disrupted, and the pelvis was gradually externally rotated. The pubic bone on the side of the sacroiliac disruption displaced inferiorly as the external rotation progressed. It is important to differentiate between the inferiorly displaced pubic bone on the side of injury in cases of external rotation injury and the superiorly displaced pubic bone on the side of injury in cases of vertically unstable pelvic injuries. This may eliminate unnecessary procedures such as skeletal traction or pinning of the sacroiliac joint.