5 hip joints with untreated congenital dislocation of the hip were examined for hip movement by ultrasound. With the hip joint flexed, the direction of the dislocated femoral head was posterior to the acetabulum. The dislocated femoral head displaced more posteriorly when the knee joint was extended, with the hip joint held in flexed position. As the dislocated hip was extended, on the other hand, the direction of the dislocated femoral head was anterior to the acetabulum. The dislocated femoral head displaced more anteriorly when the baby tried to bend the hip with the joint held in extended position. These findings indicate that a fetal posture with the hip flexed and the knee extended predisposes to the development of CDH by the action of hamstrings, and that an infant posture with the hip extended is likely to provoke femoral head dislocation by the action of the iliopsoas.