We hypothesized the extent of involvement of the proximal femoral growth plate in Perthes disease determined the final radiographic outcome after containment by shelf acetabuloplasty. We retrospectively evaluated the extent of growth plate involvement using a modified version of the method described by Yasuda and Tamura. In our modification, we used only the epiphyseal border for measurements, which was clearly visible as a thin white line, unlike Yasuda and Tamura who used the metaphyseal and epiphyseal borders. We could not clearly demarcate the metaphyseal border in the radiographs of our patient population between 1944 and 1998, which consisted of 69 patients who had surgery at a mean age of 9 years (range, 6.0-14.1 years). From these measurements, we formulated an index termed "growth plate involvement." Radiographic results were classified as described by Stulberg et al A growth plate involvement index less than 0.25 resulted in a good radiographic outcome. We found 93.2% sensitivity and 100% specificity in predicting Stulberg's outcomes. The growth plate involvement index is a reliable and reproducible measurement method and may be used prospectively as a useful prognostic factor to predict radiographic outcomes after containment acetabuloplasty.