Computed tomography (CT) and plain radiography of the pelvis were compared in 26 adult patients in whom acetabular fractures or posterior femoral head dislocations were demonstrated or suspected on initial radiographs. The detection of abnormalities of the sacrum, sacroiliac joint, iliac wing, acetabular roof, joint space, anterior pelvic column, posterior pelvic column, posterior acetabular lip, pubic rami, and quadrilateral surface of the ilium by the two methods was compared. The ability of the two techniques to establish the stable fragment was also examined. Three independent readings of each examination for each pelvic region were tabulated, and sensitivity and specificity values were calculated. There was no significant difference between plain radiography and computed tomography in detection of fractures of the iliac wing, anterior pelvic column, posterior pelvic column, and the pubic rami, with high sensitivity and specificity for both examinations. CT was more sensitive than plain radiography in detecting fractures involving the sacrum, quadrilateral surface, acetabular roof, and posterior acetabular lip. In addition, abnormalities of the hip joint space, principally loose bone fragments, were detected more often by CT. Sensitivity of both examinations for abnormalities of the sacroiliac joint was relatively poor, but examinations were highly specific. Determination of the stable fracture fragment(s) was readily accomplished by CT scanning in all 26 patients; in five patients incorrect determinations were made with conventional radiographs alone.