High tibial osteotomy is a useful procedure in the treatment of early stage osteoarthritis of the knee which is confined to the medial compartment. But long term follow-up study of high tibial osteotomy has not been provided yet and its prerequisite for a successful result still remains unrevealed. Since the osteoarthritis can progress after the osteotomy, it may result in the loss of correction and recurrence of pain and require conversion to total knee replacement arthroplasty. We studied clinically and radiologically the long term result of high tibial osteotomy in 11 patients(16 knees) who had been treated at the Department of Orthopaedic surgery of Seoul Paik Hospital during the period from January, 1978 to May, 1989, and follow-up for longer than 4 years. Preoperative mean varus angle was 6.4 degrees and immediate postoperative mean valgus angle was 8.6 degrees and final mean valgus angle was 4.7 degrees. The average loss of correction angle was 3.9 degrees. At 2-year follow-up, the results of 16 knees were as follows; excellent in six knees, good in seven knees. The final results at average 7.2-year follow-up were excellent in four knees, good in six knees and one knee was converted to total knee replacement arthroplasty. The results showed deterioration of clinical results and loss of correction with time after osteotomy. However, in more than half of the cases the results were good or excellent at the final follow-up, We, therefore, conclude that high tibial osteotomy is still a useful procedure in osteoarthritis of the knee if performed with a precise surgical technique and an adequate overcorrection more than 10 degrees.