In the treatment of DDH, generally we have used the acetabular index (AI) and the center-edge (CE) angle as diagnostic factors for hip dysplasia. However, it is sometimes difficult to mark the exact lateral edge of the acetabulum to measure these factors due to indistinct bony shadows on the image of the dysplastic hip. The purpose of this study was to evaluate the lateral edge of the acetabulum to determine the location of the most accurate marking point on the plain radiograph. Using plain radiograph, we studied 53 patients with DDH which were treated with closed reduction. 26 patients were studied using plain radiograph and another method: 16 patients with 3- DCT, 4 patients with MRI, and 6 patients with arthrograph. According to this study, the most lateral bony margin of the acetabular roof on the plain radiograph indicates the antero-lateral portion of the acetabulum. The lateral edge of the sourcil indicates the lateral margin of the centro-lateral (mid-superior) portion of the acetabulum. The marking point on the lateral end of the sourcil reflects a dysplastic condition of the acetabulum, especially in the mid-superior portion of the acetabulum.