Aims: The purpose of our retrospective case-series study was to determine factors that inßuence the long-term outcome of high tibial osteotomy (HTO). Methods: Between 1984 and 1990, 164 patients received an HTO for medial unicompartmental OA associated with varus deformity of the knee. All the HTOs were performed with a modiþed technique of Weber and Coventry. 39 knees in 35 patients were assessed at an average time of 13.5 years by an independent clinician. Patients were evaluated by the Clinical Rating System of the Knee Society, and underwent a complete radiological exam. Results: The results of the function score from patients with no further treatment were good to excellent in 71.5% of the cases and fair in only 10.7%. There were 17.8% poor results. There was a progression in radiological evidence of arthrosis after 10 to 15 years compared to the mid-term results after three to þve years. Patients with a previous meniscectomy had a statistically signiþcant worse function score (p=0.05) than patients who underwent HTO only because of malalignment induced OA. When the size of the osteotomy wedge was bigger than 20¡, there was a signiþcantly worse function score (p=0.049). Conclusion: The long-term results of this study conþrm, that treatment of varus gonarthrosis with HTO generally yield a long time (mean time of survival was 11.5 years) of pain-reduction and a good function for most of the patients.