PURPOSE: The purpose of our study was to compare clinical and radiological results of two groups of patients treated for medial compartment osteoarthritis of the knee with either conventional or computer-assisted open-wedge high tibial osteotomy (HTO). Goals of surgical treatment were a correction of the mechanical axis between 2 degrees and 6 degrees of valgus and a modification of posterior tibial slope between -2 degrees and +2 degrees . METHODS: Twenty-four patients (27 knees) affected by varus knee deformity and operated with HTO were prospectively followed-up. They were randomly divided in two groups, A (11 patients, conventional treatment) and B (13 patients, navigated treatment). The American Knee Society Score and the Modified Cincinnati Rating System Questionnaire were used for clinical assessment. All patients were radiologically evaluated with a comparative lower limb weight-bearing digital radiograph, a standard digital anteroposterior, a latero-lateral radiograph of the knee, and a Rosenberg view. RESULTS: Patients were followed-up at a mean of 39 months. Clinical evaluation showed no statistical difference (n.s.) between the two groups. Radiological results showed an 86% reproducibility in achieving a mechanical axis of 182 degrees -186 degrees in group B compared to a 23% in group A (p = 0.0392); furthermore, in group B, we achieved a modification of posterior tibial slope between -2 degrees and +2 degrees in 100% of patients, while in group A, this goal was achieved only in 24% of cases (p = 0.0021). CONCLUSION: High tibial osteotomy with navigator is more accurate and reproducible in the correction of the deformity compared to standard technique. LEVEL OF EVIDENCE: Therapeutic study, Level II.