We measured coronal alignment with mechanical tibiofemoral angle (MTFA) on the DLS and SLS radiographs in patients with posterolateral rotatory instability (PLRI, 30 knees), osteoarthritis (OA) with varus deformity who were scheduled for high tibial osteotomy (HTO) (60 knees), and in normal control (60 knees). The measurements on the SLS radiographs were compared with those on DLS images and the size of the differences were compared between the three groups. The correlation between the radiograph-related differences of coronal alignment and the limb alignment or geometry of tibia/femur was investigated. In the OA group, the size of the radiograph-related differences before HTO were compared with those after surgery.