46 patients with knee joint arthritis were subjected to arthroscopic examination. The results were analyzed and its clinical significance was discussed. The postoperative diagnoses were rheumatoid arthritis in 19 cases, Sjögren syndrome in one, seronegative spondyloarthropathy in 4, gouty arthritis in one, tuberculous arthritis in one, pigmented villous nodular synovitis in two, undefined synovitis in 5, osteoarthritis in 8 and nonsynovitis conditions in 5. The most important indication of arthroscopic examination is for differential diagnosis of arthritis of unknown cause, such as rheumatoid arthritis with single knee joint involvement, osteoarthritis with symmetric large and small joints involvement as well as crystal induced arthritis. These conditions showed under arthroscope their special features in synovium both grossly, microscopically and immuno-pathologically. The synovial damage as observed through arthroscopic examination correlated to certain extent with the X-ray film changes of the knee joint, but direct visualization with the former is better than indirect imaging with the latter.