INTRODUCTION: Osteoarthritis (OA) of the ankle with an unknown etiology (primary OA) is rare, whereas a secondary OA due to a known cause is not uncommon. OA of the knee can, when it progresses, change the alignment of the whole extremity including the ankle joint. Since we had observed in patients coming for OA knee surgery coexisting OA in the ankle joint, our objective of this study was to assess the prevalence of abnormal alignment and degenerative changes in the ankle joint and to determine if degenerative changes in the knee and ankle correlated. These aims were assessed retrospectively, utilizing mechanical axis (MA) radiographs of subjects with knee OA prior to surgery. MATERIALS AND METHODS: This retrospective study consisted of 104 patients who had undergone surgery of the knee due to OA. A musculoskeletal radiologist and an orthopedic surgeon reviewed in consensus the preoperative and postoperative MA radiographs. We analyzed and measured the tibiofemoral (TF) angle, the mediolateral tibial translation, the deviation of the MA, the tilt of the talocrural joint and the degree of OA in the knee and ankle. RESULTS: The severity of OA of the TF joint correlated with the preoperative TF translation and moderately with the deviation of the MA and abnormal TF angle. Of the 104 patients, 30 showed coexisting OA of the ankle: the greater the tilt in the ankle, the more degenerative were the changes in the joint. The MA was corrected with surgery from a mean preoperative 5.4 degrees varus to a postoperative mean 1.2 degrees valgus, and the ankle tilt was changed significantly at the same time. CONCLUSION: Ankle OA and tilt were not uncommon in our patients with knee OA. The assumption that associations may exist between knee and ankle OA and joint malalignment was confirmed.