Objective: To observe the long-term outcome of high tibial osteotomy (HTO) in treating medial compartment osteoarthrosis of knees. Methods: A retrospective study was carried out on 194 patients (215 knees) treated with HTO for medial compartment osteoarthritis at the Orthopaedic Hospital of Kiel University between 1985 and 1996. Results: One hundred and sixty-one knees (144 patients) were followed up for 1.5-12 years with an average of 7.5 years and their data were reviewed. The proportion of excellent outcome were 97.3%, 93.6% and 78.2% two, five and over five years after HTO, respectively. The revision rate of total knee arthroplasty (TKA) was 11.8% (19 knees retreated with TKA for HTO failure). The survivorship analysis of the 19 knees retreated with TKA showed an expected survival rate of 98.7%, 95.0% and 84.1% 2, 5 and 10 years after HTO, respectively. There were 5.6% complications (12 /161), including five superficial wound infections, one deep infection, five delayed bone healing, and one peroneal nerve palsy. Fifty patients (54 knees) missed follow-up, among them 10 patients (11 knees) died.Conclusions: HTO is an effective method in treating medial compartment osteoarthritis with a varus knee. Appropriate overcorrection of femorotibial alignment is the key for the success of the operation. But as the long-term effect is conced, there is a trend of deterioration and some of the patients may have a second operation of revision with TKA.