OBJECTIVE The purpose of this study was to develop imaging criteria for the diagnosis of meniscocapsular separation by correlating findings on MR imaging, MR arthrography, and sonography of normal and abnormal medial meniscocapsular structures with corresponding anatomic sections in cadavers. METHODS Eight cadaveric knee specimens were examined with MR imaging, MR arthrography, and sonography before arthroscopy. In six specimens the following lesions were arthroscopically created: meniscocapsular separation (n = 3), medial collateral ligament (MCL) tear (n = 3), tear of the meniscofemoral extension of the deep MCL (n = 2), and coronary ligament tear (n = 2). After arthroscopy, all imaging studies were repeated. The specimens were sectioned for correlation with imaging studies. RESULTS MR findings that correlated with meniscocapsular separation were interposition of fluid between the meniscus and the MCL, irregular meniscal outline, and increased distance between the meniscus and the MCL. On MR arthrography meniscocapsular separation correlated with interposition of contrast medium between the meniscus and the MCL. Tears of the meniscofemoral extension of the deep MCL were best shown on MR arthrography. Sonography showed deep and superficial MCL lesions but did not show meniscocapsular separations. CONCLUSIONS In arthroscopically created meniscocapsular separation, the lesion is suggested on MR images when fluid is interposed between the meniscus and the MCL, when the meniscal outline is irregular, or when the distance between the meniscus and the MCL is increased. On MR arthrograms, a meniscocapsular separation is suggested when contrast medium is interposed between the meniscus and the MCL. Sonography does not allow accurate diagnosis of meniscocapsular separation.