Fifty knees that had arthroscopic surgery for treatment of osteoarthritis were reviewed, and prognosis evaluated corresponding to articular cartilage damage. Surgery was performed including arthroplastic meniscal suture for meniscal tear, or if this was impossible, meniscectomy, articular cartilage smoothing for cartilage loss, osteophyte resection for obstructing osteophytes, enucleation for loose body, lateral release for patellar lateral shift, and synovectomy for hydrops. Preoperative HSS score (p<0.0001) increased in all cages. Only medial articular cartilage loss significantly (p<0.05) influenced post-sugery prognosis. Other factors, such as lateral articular cartilage loss, patello femoral articular cartilage loss, medial and lateral meniscal damage had no significant effect on the postoperative prognosis. There fore, careful attention must be paid to the medial compartment for prediction of osteoarthritis of the knee, when arthroscopically observed.