Osteopetrosis is a hereditary disease characterized by extraordinary thickness and densityof the cortical portions of the osseous system. The most likely explanation for the pathopysiology is that there is functional defect in the osteoclasts, which renders them unable to resorb bone and cartilage normally. Secondary manifestations of osteopetrosis are multiple fractures, epiphyseal deformities, retarded development, myelophthisic anemia, etc, but hip dislocation is not included herein. The authors have experienced three cases of osteopetrosis which were found unusually to have hip dislocation(g), and those cases are to be reported with exhaustive review of references.