PURPOSE: Preoperatively planned correction for tibial osteotomy surgery is usually based on weightbearing long-leg Xrays, while the surgery is performed in a supine non-weightbearing position. The purpose of this study was to assess the differences in lower limb alignment in three different weightbearing conditions: supine position, double-leg (DL) stance and single-sleg (SL) stance prior to performing a medial opening wedge high tibial osteotomy (MOWHTO) for varus malalignment. The hypothesis of this study was that progressive limb-loading would lead to an increased preoperative varus deformity. MATERIAL AND METHODS: This retrospective study included 89 patients (96 knees) with isolated medial knee osteoarthritis (Ahlbäck grade I or II) and significant metaphyseal tibial vara (\textgreater 6°). The differences between supine position, DL stance and SL stance were analysed for the hip-knee-ankle angle (HKA), lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA), weight-bearing line ratio (WBL) and joint line convergence angle (JLCA). RESULTS: From a supine position to DL stance, the HKA angle slightly increased from 175.5° ± 1.1° to 176.3° ± 1.1° and JLCA changed from 2.0° ± 0.3° to 1.8° ± 0.3° without a statistically significant difference. From DL to SL stances, the HKA angle decreased from 176.3° ± 1.1° to 174.4° ± 1.1° (p \textless 0.05) and the JLCA increased from 1.8° ± 0.3° to 2.6° ± 0.3° (p \textless 0.05). A significant correlation was found between ΔHKA and ΔJLCA between the DL and the SL stances (R(2) = 0.46; p = 0.01). CONCLUSION: Varus malalignment increases with weight-bearing loading from double-leg to single-leg stances with an associated JLCA increase. Thus, single-leg stance radiographs may be useful to correct preoperative planning considering patient-specific changes in JLCA. LEVEL OF CLINICAL EVIDENCE: III, retrospective comparative study.