Background: The purpose of the current study was to upgrade the International Hip Dysplasia Institute (IHDI) classification of developmental dysplasia of the hip (DDH). Methods: The upgrading was suggested by adding the state of the acetabulum (type A for the normal acetabulum and type B for the dysplastic one). The pelvic radiographic films of 110 children suspected to have DDH were used by three observers to sort out the hips into grades according to the original form and the suggested upgraded one subsequently. Results: The interobserver reliability between the observers improved from a good level (intraclass correlation coefficient [ICC], 0.885; 95% confidence interval [CI], 0.856–0.909) with the original form to an excellent level (ICC, 0.919; 95% CI, 0.898–0.936) with the upgraded form. When the upgraded form was used, only the grade 1 hips were divided into types A and B, while those classified as grades 2, 3, and 4 were all graded as type B only. Conclusions The IHDI classification of DDH can be upgraded into grade 1A, grade 1B, grade 2, grade 3, and grade 4.