BACKGROUND The etiology of primary osteoarthritis of the ankle is not well understood. The varus deformity of the ankle and the compensatory function of the subtalar joint are expected to be important factors in the progression of osteoarthritis. The purpose of this study was to document joint inclination at the various stages and to try to deduce the mechanism of primary, varus osteoarthritis of the ankle. METHODS Weightbearing ankle anteroposterior (AP) views, lateral views and subtalar views were taken for 133 ankles in 80 patients (OA group) and 62 ankles in 50 subjects (control group). The OA group was divided into 5 radiographic stages. The inclination of the articular surface of the tibial plafond, the talar dome and the posterior facet of the calcaneus were measured from the radiographs. The inclination of the subtalar joint against the ankle was also calculated. At each stage, each value of the OA group was compared with the control group. RESULTS The varus inclination of the articular surface of the tibial plafond progressed by stages; however, anterior opening was not significant at all stages. The valgus inclination of the subtalar joint progressed until the intermediate stage and converted to varus position at the later stage. CONCLUSION The compensatory function of the subtalar joint was most pronounced at the intermediate ankle arthritis stage.