In 53 cases of injury of the lumbosacral plexus, 31 were due to trauma and 22 followed operations on the hip joint. Post-traumatic lesions occur mostly in conjunction with severe bony injuries, especially fractures of the acetabulum and of the pelvic ring. Nearly always, it is the sacral portion of the plexus that is involved, either predominantly or exclusively. Seventeen of the 22 postoperative pareses followed complete hip-joint replacement. In the postoperative lesions the lumbar plexus portion is most frequently involved. Ninety-one percent of all of our cases were misdiagnosed in previous clinical examinations, that is, as a lesion of the femoral or sciatic nerve, or they were unrecognized because of lack of awareness of the possibility of plexus damage or because the signs were obscured by the associated bony injuries or hip-joint disease. In order to make an exact diagnosis, a detailed electromyographic investigation is necessary.