Realignment knee osteotomy relies on accurate preoperative assessment of coronal alignment. Weightbearing (WB) 'long-leg' (LL) radiographs are the accepted gold-standard investigation, though in practice standard knee radiographs (short leg; SL) and non-weightbearing (NWB) cross-sectional imaging such as computed tomography (CT) scanograms have been used. We compare the accuracy of SL and NWB radiographs to formal LL alignment radiographs. A prospectively maintained osteotomy database was reviewed to identify the study population. All patients underwent standardised weightbearing long-leg alignment radiographs. The series was screened consecutively until 30 patients who also underwent WB SL radiographs ('WB cohort'), and 30 with NWB SL ('NWB cohort') radiographs, were identified. Anatomic tibiofemoral angle was calculated by independent reviewers using a validated technique from both radiographs and contrasted. 60 patients were identified as outlined in the study protocol. There were no differences in baseline demographics. Coronal alignment calculated from SL and LL radiographs differed significantly (median difference 2.1°, p < 0.001). Alignment values from weightbearing SL radiographs demonstrated markedly greater agreement with LL values than those from NWB radiographs (intraclass correlation coefficient 0.878 vs 0.657), with the NWB cohort also exhibiting greater outlier and extreme outlier incidence. Our data adds to the growing evidence that SL radiographs are inadequate in the interpretation of knee alignment. In addition, we demonstrate that NWB radiographs (and by extension other NWB modalities such CT scanograms) demonstrate poorer agreement to gold-standard than WB methods. Coronal alignment of the knee cannot be reliably measured from non-weightbearing imaging modalities. Though potentially useful as an adjunct, non-weightbearing cross-sectional imaging and standard knee radiographs should not be used as a proxy for formal weightbearing long-leg radiographs in osteotomy planning. Copyright © 2020 The College of Radiographers. Published by Elsevier Ltd. All rights reserved.