A prospective double-blind study was undertaken to evaluate the usefulness of magnetic resonance imaging in the accurate interpretation of pathological intra-articular changes in the knee. Forty-seven patients who were scheduled to have arthroscopy and three patients who wer to have arthrotomy volunteered for magnetic resonance imaging preoperatively. The radiologists had no clinical or roentgenographic information about the patients before the evaluation of the magnetic resonance images, and the radiologists' interpretations were unknown to the surgeon before the arthroscopy or arthrotomy was done. Our important observations were limited to the findings in the menisci and in the anterior cruciate ligament. Magnetic resonance imaging had a positive predictive value of 75 per cent, a negative predictive value of 90 per cent, a sensitivity of 83 per cent, and a specificity of 84 per cent for pathological findings in the menisci. For complete tears of the anterior cruciate ligament, the positive predictive value was 74 per cent; the negative predictive value, 70 per cent; the sensitivity, 61 per cent; and the specificity, 82 per cent. We believe that magnetic resonance imaging, when combined with clinical and roentgenographic examination, provides the most accurate non-invasive source of information that is currently available for pathological findings in the menisci and in the anterior cruciate ligament.