This prospective double-blind study was designed to evaluate the capability of magnetic resonance imaging to serve as a diagnostic tool in patients who have a clinically suspected disorder of the meniscus. The imaging studies provided a diagnostic accuracy of 72 per cent, a sensitivity of 88 per cent, and a specificity of 57 per cent. The positive and negative predictive values were 66 and 83 per cent. The diagnostic sensitivity was 94 per cent for lesions of the medial meniscus; this value differed significantly from that of 78 per cent for lesions of the lateral meniscus (p less than 0.05). The 37 per cent specificity for lesions of the medial meniscus was extremely low compared with the rate of 69 per cent for lesions of the lateral meniscus (p less than 0.01). In the intermediate part of the meniscus, the diagnostic sensitivity was 37 per cent on the medial side and 23 per cent on the lateral side; these values were significantly less than the average of 74 per cent for the other meniscal segments (p less than 0.001). The imaging studies provided an over-all accuracy of 67 per cent in the detection of degeneration of the meniscus, 78 per cent in the identification of meniscal tears, and 82 per cent in the delineation of postoperative lesions.