Routine radiographs in the follow-up study of Legg-Calvé-Perthes disease patient have generally been plain radiographs of the anteroposterior and frog-leg lateral view. Recently, several reports have found that osteochondritis dissecans can develop in 2% to 4% of Legg-Calvé-Perthes disease patients. Early roentgenographic recognition of this lesion is very important for its long-term prognosis. However, follow-up examination of the lesion using plain radiography is sometimes not satisfactory in terms of delineating the lesion. Accordingly, the authors performed computed tomography and three-dimensional reconstruction in 13 hips with osteochondritis dissecans following Legg-Calvé-Perthes disease and were able to obtain more precise information on the extent of the involvement, the degree of healing, the stability of the osteochondral fragment, and the location of the dislocated loose body.