The authors analyse peri-prosthetic ossification, a complication which can occur in the presence of certain risk factors. Grades I and II calcifications are frequent and do not interfere with the final post-operative result. On the other hand, grades II and IV result in a progressive stiffening of the coxo-femoral joint within one year after the operation. They may be bilateral and can sometimes be responsible for serious handicap. These cases should be re-operated in order to restore joint mobility, but surgical treatment should be completed by radiotherapy according to a defined protocol. Non-steroidal anti-inflammatory agents and diphosphonates also have a preventative action. Subjects at risk (ankylosing vertebral hyperostosis, spondyloarthritis, alcoholism, patients with a history of ossification of the first side) require effective prophylactic treatment. In the authors' own series, 49 cases of ossification were observed in a retrospective analysis of 100 patients, including 6 cases of grades III and IV ossification. They analyse their results in relation to the initial hip pathology treated by prosthesis, but their series is too small to allow an statistically valid conclusions. Complementary studies are required.