We evaluated the midterm results of high tibial osteotomy in patients with medial compartment arthritis. This study included 53 patients treated with high tibial osteotomy accompanied by either internal fixation (group A, n=26) or Ilizarov-type external fixator (group B, n=27). Clinical assessment of patients was performed using Hospital of Special Surgery scoring. Radiography was based on orientation angles of the knee (medial proximal tibial angle, lateral distal femoral angle, posterior proximal tibial angle) and mechanic axis alignment test. We used the Insall-Salvati index to determine patellar height. The mean follow-up in group A was 77 months (range 63118) and that in group B 72 months (range 61-113). The patients of group B demonstrated better results in terms of Hospital of Special Surgery score, alignment of lower extremity, and preventing of progression of arthritis. We believe that normal alignment and orientation of the knee could be precisely established with a circular external fixator, and that some disadvantages such as patella infera, failed correction, and loss of bone in proximal tibia as observed after classic high tibial osteotomy can be avoided. Furthermore, progression of arthritis can be prevented if the mechanical axis passes through the lateral compartment of the knee.