Background: Undetected developmental dysplasia of the hip can progress to hip instability, which can lead to the development of early osteoarthritis. The purpose of the present study was to determine the reliability and accuracy of the Shenton line in the diagnosis of acetabular dysplasia in the skeletally mature patient. Methods: Supine anteroposterior pelvic radiographs were obtained for a total of 128 hips in patients with a diagnosis of developmental dysplasia of the hip prior to pelvic osteotomy (sixty-four hips, including thirty-two left and thirty-two right hips) and normal patients (sixty-four hips, including thirty-two left and thirty-two right hips). Six orthopaedic surgeons determined if the Shenton line was broken or intact to determine interobserver reliability. Four observers reviewed the randomized radiographs again after four weeks to determine intraobserver reliability. Results: The mean intraclass kappa value was 0.90 (range, 0.79 to 0.97), indicating excellent agreement between each reviewer's primary and secondary reviews. The kappa estimate for interobserver reliability among all six reviewers was 0.80 (95% confidence interval, 0.75 to 0.84), indicating an excellent agreement. The determination of a normal or abnormal Shenton line had a mean sensitivity of 83.3% (range, 60.9% to 93.8%) and specificity of 98.4% (range, 92.2% to 100%). Conclusions: The Shenton line is a reliable and accurate radiographic marker to detect superior femoral head subluxation indicative of acetabular dysplasia.