A retrospective study was made on 536 patients in whom a Charnley-Muller total hip prosthesis had been inserted. Two hundred and seventeen cases were available for follow-up, 121 of them having signs of diffuse idiopathic skeletal hyperostosis (Forestier's Disease) which is common in the elderly. Peri-prosthetic ossification was found in 46 p. 100 of patients with hyperostosis and in only 24 p. 100 of patients without it (p = less than 0,01). In both groups the ossification was more frequent in men than in women (p = less than 0,001 in the hyperostosis group and less than 0,03 in the controlled group). In cases of bilateral prostheses, ossification was identical on both sides in 81 p. 100 of cases. Only 66 p. 100 of the men with hyperostosis obtained satisfactory postoperative hip mobility as against 83 p. 100 of men in the control group (p = 0,05). The difference was not significant in women. Pain was relieved in 93 p. 100 of patients with hyperostosis with no significant difference from the control group. It is concluded that the existence of diffuse idiopathic skeletal hyperostosis is not a contra-indication to total hip replacement provided that the main complaint of the patient is of pain.