Osteochondritis dissecans is a lesion of articular surfaces that is of uncertain etiology. These lesions are seen on radiographs as a bony defect or fragmentation of the subchondral bone. A bony defect may be an actual surface hole or the defect may be filled with fibrous tissue or fibrocartilage. Similarly, the apparent bone fragments may be only partially attached so they are unstable and prone to displacement or they may be firmly attached with fibrous tissue. Knowledge of fragment stability and the presence of an articular cartilage defect is useful in deciding on treatment. This information cannot be determined on plain films or clinical examination. We correlated MR examinations with arthroscopic findings in 21 patients with osteochondritis dissecans of the knee to see if MR imaging could be used to predict lesion stability and articular cartilage defects. A high-signal interface between the lesion and the femur was used as evidence of lesion instability and was found in 15 lesions. One of these lesions was questionably stable at surgery; the remainder were unstable and partially attached. The other six patients had displaced fragments with large articular defects that were clearly visualized on the MR examinations. We conclude that MR imaging is useful in evaluating articular surface defects and lesion stability in patients with osteochondritis dissecans.