Although heterotopic ossification following total hip arthroplasty is frequently observed radiographically, it fortunately is much less commonly of clinical importance. There are, however, a group of patients that develop significant heterotopic bone formation, which can be symptomatic and, in some cases, can require repeat surgery. Careful surgical technique seems indicated in all patients undergoing total hip arthroplasty to try to reduce the incidence of this problem. Patients who are recognized to be at risk should be treated with prophylaxis. These include patients with active ankylosing spondylitis, skeletal hyperostosis, and prior heterotopic bone formation. Both low-dose radiation and nonsteroidal anti-inflammatories have been shown to be effective. The choice between these two modalities depends on the patient's individual circumstances, the availability of radiotherapy support (including custom shielding for ingrowth components), and the presence of areas of bone grafting and any osteotomies or fractures. Nonsteroidal anti-inflammatories, particularly Indomethacin, are a very acceptable form of prophylaxis and may be preferred in certain patients, including young women of childbearing age. Radiation is preferred in those patients with known GI intolerance to these medications or with a prior history of peptic ulcer disease. Excision of heterotopic bone, if symptomatic, should not be performed before 6 to 12 months and then only once it is clear that the process is mature. Bone scans can be helpful in assessing the maturity of heterotopic bone and can guide the timing of excision. Prophylaxis should be carried out in all patients following excision of heterotopic bone.(ABSTRACT TRUNCATED AT 250 WORDS)