Little attention has been paid to the various causes preventing or blocking a concentric reduction following traumatic dislocation of the hip without apparent fracture. In a review of fifty-four dislocations, nine required open reduction after either a failed attempt at closed reduction or a non-concentric reduction. The factors preventing closed reduction included the femoral head buttonholed through the capsule and the piriformis muscle displaced across the acetabulum. A concentric anatomical reduction was prevented because of an inverted lumbus or an osteocartilaginous loose body in the acetabulum. Guidelines for diagnosis and treatment are discussed.