There are several diagnostic methods to evaluate the internal derangement of the knee joint. Among them, magnetic resonance imaging is a recently developed diagnostic tool and has increasing tendency. Magnetic resonance imaging can show osseous and soft tissue structures without the use of ionizing radiation, and it is not invasive. From Jan. 1990 to Feb. 1992, concomitant MRI and arthroscopy were performed in 51cases of internal derangement of the knee. There were 39 meniscus tears and 18 cruciate ligament injuries by arthroscopic diagnosis. Compared with arthroscopy, the sensitivity, specificity, and accuracy of magnetic resonance imaging were 87.5%, 81.5% and 84.3% for medial meniscus, 70.6%, 79.4% and 76.5% for lateral meniscus, 81.3% 88.6% and 86.3% for anterior cruciate ligament, 100%, 98.0% and 98.0% respectively for posterior cruciate ligament. Several authors have reported on the usefulness of magnetic resonance imaging of the internal derangement of the knee joint. We believe that magnetic resonance imaging, when combined with clinical examination, provides the most accurate non invasive source of information for pathological findings in the menisci and cruciate ligament. It is also valuable for efficient preoperative planning of the internal derangement of the knee.