The sensibility and specificity of MRI diagnosis of meniscal tears are still debated. We compared the radiologic and arthroscopic diagnosis of 37 patients who underwent MRI study of the knee, followed by arthroscopic surgery. Meniscal lesions were classified according to their size and the grading system of Lotysch and Crues (1). In patients without previous arthroscopic surgery (n = 32), 92% of the grade-3b lesions are found to be torn at surgery. The probability for tears is weaker for grade-3a lesions (one meniscus torn out of three). An intact or slightly degenerated meniscus on MRI (grades 1 and 2) relates in 91% of cases to a normal fibrocartilage at arthroscopy. In patients with previous knee surgery (n = 5), the specificity of MRI for grade-3b lesions is weak (55%), owing to the presence of granulation tissue. Our results agree with those of recent publications, confirming the high value of MRI in the diagnosis of traumatic or degenerative meniscal tears. The use of a grading system incorporating the size of the lesion gives a better specificity to MRI diagnosis.