In order to clarify the changes seen in the new acetabular roof produced by Chiari pelvic osteotomy, we performed histological investigations post-operatively, as well as arthrography and arthroscopy before and after the operation. Plain X-rays showed an osseous step between the original surface and the new acetabular roof. Arthrography demonstrated no step between the original acetabular cartilage and the surface of the newly formed acetabular roof. At arthroscopy, the surface of the new acetabular roof could be seen to be covered by a soft white tissue. Histological specimens were taken from the new acetabular roof. Examination revealed that the superficial layer consisted of some oval shaped cells with an extracellular matrix slightly stained by safranin O, but not by anti-human type II collagen antibodies. In the deepest layer, there was cartilage tissue consisting of cartilage-like cells and an extracellular matrix which was stained by safranin O and by anti-type II collagen antibodies. However, the staining intensity of this cartilage tissue was less than that of hyaline cartilage, and the direction of collagen fibers could be clearly seen. These results suggested that the tissue covering the new acetabular roof had no ability to change into hyaline cartilage, despite the superficial layer creating a smooth surface. It was suggested that the bony modification of the new acetabular roof resulted from enchondral ossification of the deepest layer.