Magnetic Resonance Imaging(MRI) is a recently developed diagnostic tool that provides multiplanar, high quality images of the body structures, and is increasingly used in evaluation of various musculoskeletal disorders. From May 1988 to July 1991, preoperative MRI and arthroscopy were performed in 84 cases of internal derangement of the knee, and their results were analysed to determine the diagnostic accuracy and clinical application of MRI. There were 66 meniscus tears and 17 cruciate ligament injuries by arthroscopic diagnosis. MRI showed high diagnostic value compared with arthroscopic findings. The accuracy, sensitivity and specificity of MRI were 94.0%, 94.4% and 93.8% for the medial meniscus, 94.0%, 90.0% and 96.3% for the lateral meniscus, 92.9%, 86.7% and 94.2% for the anterior cruciate ligament, 98.8%, 100% and 98.8% for the posterior cruciate ligament. In spite of its expensiveness, the MRI was safe, non-invasive and highly sensitive diagnostic modality. In internal derangement of the knee, when the pathologic lesion cannot be accurately diagnosed with history and physical examination, MRI can be an excellent diagnostic modality, and it is also helpful for optimal surgical planning of the clinically suspected knee injuries.