High tibial osteotomy (HTO) was performed on 86 medially osteoarthritic knees in 78 patients. Twelve patients died during the follow-up period. Fifty-six knees of 51 patients were evaluated twice postoperatively: once at six years after surgery and again after ten to 15 years. Results were satisfactory in 88% of the knees at the six-year follow-up evaluation and in 63% at the post-ten-year follow-up evaluation. Ten-year follow-up results significantly deteriorated. The femorotibial angle (FTA) at the time of bone union after HTO was a significant factor that contributed to the ten-year results. In patients maintaining excellent improvement of knee function at the post-ten-year follow-up evaluation, the mean FTA at one year after surgery was 165 degrees; this value was constant through ten years after HTO. An FTA of 164 degrees-168 degrees should be attained to ensure favorable long-term results in HTO.