The authors report 7 cases in which removal of high grade ossification which developed after hip arthroplasty was performed: all of the patients were treated postoperatively by radiation therapy with the purpose of preventing recurrence of ossification. After surgery ossification was present, but to a moderate degree, in 6 out of 7 cases examined; movement had clearly improved. The results obtained show that removal surgery is justified if movement of the hip is considerably impaired (ankylosis) and that prevention plays an essential role and must be carried out every time that the patient risks the development of ossification.