It is postulated that the variable clinical and radiological appearances of coxarthrosis are related to its pathogenesis and are important in deciding the choice of treatment. In a study of 150 patients the hip disorder was classified according to the presence or absence of some local anatomical defect which could predispose to mechanical overload, and also according to the presence or absence of some primary disorder which could cause cartilage degeneration. In addition, new bone formation and remodelling were assessed: this reparative response was most marked in joints with anatomical abnormalities ("hypertrophic OA") and least evident in those with previous inflammatory disease ("atrophic OA"). This study suggested that the behaviour of coxarthrosis is determined by three interacting factors: (1) cartilage degeneration, (2) excessive mechanical stress, and (3) the reparative bone response. The prognosis of the various types and the implications for treatment are discussed. Where anatomical abnormalities and mechanical features are dominant, cartilage loss is at first localised, remodelling is usually good and the hip can stabilise; in these cases osteotomy is often successful and prosthetic fixation is likely to remain secure. Where inflammatory and degenerative features predominate, reparative new bone formation is minimal and progression is more rapid; osteotomy is much less likely to be effective in these cases.