From January 1976 to December 1990, 66 high tibial osteotomies for medial gonarthrosis were performed in 59 patients. Thirty knees of 26 patients (18 women and 8 men) were reviewed after a mean follow-up period of 15.3 years (range, 10-24 years). The mean age of these 26 patients was 59 years at the time of operation and 75.5 years at the latest follow-up. The average femoro-tibial angle changed, from 187 degrees before surgery to 170 degrees after surgery, and to 174 degrees at the latest follow-up. Progression of osteoarthritic changes was observed in 87% for the medial and in 90% for the lateral compartment of the knee. Clinical results, assessed according to the Japanese Orthopaedic Association (JOA) knee score, showed improvement with a score of 65 points before surgery, and a score of 81 points at the latest follow-up. The overall clinical results were satisfactory for 60% of the knees. The results of this long-term follow-up study show that high tibial osteotomy for medial compartment osteoarthritis can be effective for as long as 15 years.