High tibial osteotomy is a satisfactory surgical method of treatment for knees with unicompartmental osteoarthritis and angular deformity. This procedure has been found to reiieve pain, improve function and delay the progress of the condition by realigning the joint and thereby shifting the line of weight-bearing to the compartment with preserved articular cartilage. We reviewed clinically and radiologically the long-term results of the high tibial osteotomy performed on 17 medially osteoarthritic knees in 15 patients at Seoul National University Hospital from October 1980 to June 1989. We excluded the cases whose medical records were not complete or the follow-up was less than 5 years. There were 14 females and 1 male, and the average age of the patients were 59 years(range, 50 to 67 years), and the mean follow-up period was 7.3 years(range, 5 to 13.1 years). The clinical results were evaluated using Insall knee-rating scale. Excellent or good result was obtained in 14 knees(82%) at 2- and 5-year follow-up and in 13 knees(76%) at the final follow-up evaluation. Preoperative mean varus angle was 6.6 degrees. Postoperative corrected tibiofemoral angle decreased with time; immediate postoperative and final follow-up valgus angles were 10.2 and 8.5 degrees respectively. The clinical results based on the valgus angle achieved immediately after operation was 75 points(below valgus 10 degrees), 84 points(valgus 10 to 14 degrees), and 76 points(above valgus 15 degrees). Preoperative radiologic stage of osteoarthritis was evaluated using modified Ahlback classification(range, stage I to IV). The knee rating score at the final follow-up was 82 points in grade I(5 knees), 80 points in grade II(6 knees), 77 points in grade III(5 knees) and 72 points in grade IV(1 knee) degenerative arthritis. This study revealed that the best results were seen in knees with preoperative grade I or II degenerative arthritis and with 10 to 14 degrees of valgus immediately after osteotomy. Although there were some deterioration of clinical results and loss of correction with time after osteotomy, we concluded that appropriate selection of patients and careful surgical technique will provide good to excellent long-term results of high tibial osteotomy.