Errors in determining the acetabular index can be induced either by incorrectly positioning the child for radiographs or by inter- or intraobserver errors. From postmortem radiographic studies, we have determined the magnitude of these errors. The error caused by pelvic rotation is +/- 3 degrees if the obturator foramina ratio is kept within 0.5 to 2. If pelvic flexion/extension is confined to +/- 10 degrees, the error induced by flexion/extension is +/- 3 degrees. The intraobserver error was +/- 2 degrees, and the interobserver error was +/- 3 degrees. Under these circumstances, the total error is +/- 5 degrees. We have not been able to find a satisfactory way of limiting the flexion/extension to +/- 10 degrees. In some circumstances, particularly if a child is distressed, the flexion/extension may be 20 degrees from neutral; under these circumstances, errors as large as 10 degrees can occur. Surgeons should be aware that very large errors can occur when the acetabular index is measured.