Conflicting opinions exist to what extent interpreters of an acute anterioposterior (AP) radiograph of the pelvis underdiagnose pelvic injury. We have prospectively evaluated the usefulness of a plain AP radiograph of the pelvis in detecting injuries, using CT as the gold standard. Seventy-three haemodynamically stable blunt trauma patients, who had an AP radiograph of the pelvis and an abdominopelvic CT as part of their initial imaging evaluation, were included. There were 14 women and 59 men, with a mean age of 30 (3-61) years. Two senior radiologists, with special interest in CT examinations evaluating trauma, reviewed the radiographs and a month later the CT scans, blinded and independently. Usefulness index of the plain AP radiograph of the pelvis in detecting specific injuries was calculated using CT as the gold standard. A radiograph is regarded as useful if the usefulness index is 0.35 or more. The usefulness index for a plain AP radiograph of the pelvis for detecting a fracture of iliac wing and os sacrum was 0.25 and 0.01, respectively. An acute AP radiograph of the pelvis is not useful in detecting a fracture of iliac wing or os sacrum.