A new classification for osteoarthritis of the knee associated with varus deformity is presented. This classification is derived from the combination of conventional radiographs, stress radiographs (when needed), and clinical examination.This study included the analysis of coronal alignment on full-leg standing radiographs of 526 patients awaiting knee arthroplasty for varus deformity in a single institution. Various mechanical and anatomic angles were measured, and these findings were combined with a basic clinical examination of patients. The radiographs were measured on 2 separate occasions to determine the intraobserver reliability. Cross-sectional studies such as computed tomography or magnetic resonance imaging were used to further refine observations about different wear patterns.Varus deformity can either be intra-articular or extra-articular. Intra-articular deformities can be correctable or fixed. In fixed deformities, the status of the lateral ligament is taken into account. Extra-articular deformity can be metaphyseal or diaphyseal, and the possibility for intra-articular correction will depend on the degree of deformity and its distance from the joint.This new classification allows for better definition of varus deformity, which can help surgeons during preoperative planning, particularly with their choice of implant and potentially the degree of constraint. The classification can also be a tool for further prospective studies about varus deformity.