20 patients who suffered from osteochondrosis dissecans in the knee were examined by x-ray and MRI. Afterwards all joints were evaluated by arthroscopy. On plain x-ray the judgement of joint cartilage is not possible. With the use of MRI, the hyaline cartilage can be documented. Even without any visible changes on x-ray, MRI clearly reveals pathologic bone patterns. Comparing areas of sclerosis on x-ray with MRI changes, the real diameter of osteochondrosis dissecans can only be seen on MRI.