To evaluate the radiographic characteristics of osteochondritis dissecans (OCD) and useful parameter for predicting mechanical stability, we retrospectively analysed 26 plain radiographic examinations and seven MR imagings in 28cases of OCD in 24 patients. Typical radiologic findings were osteochondral defect with sclerotic rim of variable thickeness and osteochondral fragment. Sites of osteochondral defect were medial (35.9%) or lateral (32%) femoral chondyle and medial (7.1%) or lateral (25%) side of talar dome. Sclerotic rim was seen in 24 cases (85%) and osteochondral fragments including nine loose bodies were seen in 21 cases (75%) The size of osteochondral defect wtith unstable fragment (average 2.05cm) and loose body (2.04cm) in the knee joint were similar to, but statistically larger than that with stable fragment (1.35cm) All osteochondral defects were well visualized on MR images. Abonrmalities of articular cartilage and effusion in the interface betwen the parent bone and fragment werw seen in five cases of which there were confirmed three unstable cases arthroscopically. We conclude that size of defect may be a good parameter for predicting mechanical stability and MRI may be useful in the diagnosis of OCD and determining the methods of treatment