Forty-eight knees in 44 patients who had a high tibial osteotomy performed for medial compartment osteoarthritis were reevaluated twice, once at 1 year after surgery and again at 10 to 15 years after the high tibial osteotomy, to determine the effects of high tibial osteotomy on progression of medial and lateral joint arthrosis. Radiologically, although arthrosis significantly increased in the medial and the lateral joint compartment, there was no significant difference in the degree of progression between the two compartments. There was a significant correlation between the progression of medial joint arthrosis evaluated at 10 years or more after surgery and the femorotibial angle measured at 1 year. There was no significant correlation between the progression of lateral joint arthrosis in 10 to 15 years and the femorotibial angle measured at 1 year. The average knee function score improved significantly from 59.1 +/- 5.5 points before surgery to 85.1 +/- 6.1 points at the 1-year followup. At the final followup, the clinical score (80.7 +/- 5.4) had deteriorated relative to the 1-year results but still was significantly better than the preoperative score. The current results indicate that the greater the surgical valgus correction, the slower the progression of medial joint arthrosis. Lateral joint arthrosis did not progress more quickly after high tibial osteotomy, even if an overcorrection was performed.