The aim of the current study was to document regeneration of the articular cartilage after correction of varus deformity for osteoarthritis of the knee. The repair of articular cartilage after high tibial valgus osteotomy for medial compartmental osteoarthritis was observed in 146 knees of 115 patients. The mean age of the patients at osteotomy was 65+/-7 years (range 47-80). Observations were made through an arthrotomy at the time of removal of the blade plate, an average of 2 years after the initial osteotomy. Regeneration of the degenerated cartilage was classified into three stages. There were no regenerative findings (Stage A) in 13 knees; partial regeneration with fibrocartilaginous tissue (Stage B) in the previous degenerated area (Stage B) in 86 knees; and total coverage by new regenerated fibrocartilage or hyaline-like cartilage (Stage C) in 47 knees. Mature regeneration (Stage C) was observed more frequently in the knees with increased width of the medial joint space after high tibial osteotomy than in those with unchanged width (P<0.01), and in the knees with more than 5 degrees of anatomical valgus angulation after osteotomy than in those with less than 5 degrees (P=0.05). Even the knees with eburnation of subchondral bone in the weight-bearing portion showed Stage C regeneration in 26 out of 80, in contrast with no regeneration in two knees with degenerated cartilage remaining. O'Driscoll's histological and histochemical grading score was higher in the Stage C group (P<0.02). The weight-bearing portion of the medial femoral condyle with exposed bone was concluded to be partially or entirely covered by newly regenerated cartilage after 1 year after adequate correction of varus deformity by high tibial osteotomy without antologous chondrocyte implantation.