The aim of this study was to describe the deformity of both bone and soft tissues in valgus knees and attempt, with the help of computer-assisted total knee arthroplasty (CAS-TKA), to answer the question whether a medial or lateral approach should be used in a fixed deformity. This was a prospective study of 52 consecutive patients with valgus knee osteoarthritis (OA) on who CAS-TKA was performed between 2008 and 2012. Only patients (N = 42) with valgus deformity Krackow type II and III were included for detailed analysis: 17 were operated utilizing a medial and 25 using a lateral approach. Computer navigation was used to perform quantitative control of alignment and gap changes in the medial and lateral knee compartment after each step of the release. Radiological alignment as well as functional results with Knee Society and WOMAC scores were evaluated after a mean (SD) of 23 (5) months in all patients. Twenty-eight different combinations of pathological changes of bone and soft-tissues were described in the 52 patients making the identification of a typical pathological pattern for valgus deformity impossible. In 60% of patients, irrespective from the approach, the valgus deformity was reduced after release of the ilio-tibial tract. No significant difference between both groups was observed in the extent of releases, gap balancing, surgical time, implants constraint, leg alignment and mid-term functional outcomes. None of the included patients were revised for any reason.