High tibial osteotomy was performed for medial gonarthrosis in 28 patients. Preoperatively, and at 3, 12, and 24 months after surgery, clinical and radiographic examinations were made, and joint-fluid samples were aspirated. Arthroscopy was performed preoperatively and at 24 months. Immunoassay of proteoglycan epitope in joint fluid showed an increase in concentration at all times as compared with a reference population with normal knee joints. An increase in both the concentration and the total amount of proteoglycan epitope in joint fluid was noted at 3 months postoperatively with a return to preoperative values at later times. Regrowth of fibrocartilage did not correlate with proteoglycan epitope data.