A typical synovial osteochondromstosis within the both knee joints occured in a 29-year-old man. The patient complained of palpable masses in the both knee joints showed multiple round, oval, amorphous calcified loose bodies. Arthrotomy of the left knee joint, removal of loose bodies and total synovectomy were performed. The microscopic findings of loose body revesled a degenerated cartilage cap and showed ares of cacification, ossification and fibroadipose tissue in the central part of the body. The mocroscopic findings of synovium revesled a cartilagenous metaplasis of the fibro vascular tassue and showed no evidence of calcification or ossification. The cartilagenous tissue was relatively matured form.