PURPOSE: To evaluate the frequency, number and signal intensity of fluid-fluid levels of musculoskeletaldiseases on MR images, and to determine the usefulness of this information for the differentiation ofmusculoskeletal diseases. MATERIALS AND METHODS: We retrospectively reviewed 30 cases in which fluid-fluid levelwas seen on MR images; they were diagnosed histopathologically(24/30), clinically(5/30) orclinicopathologically(1/30). To differntiate these diseases, we determined their frequency, the number offluid-fluid levels and the signal intensity of fluid. RESULTS: MR images revealed fluid-fluid levels in thefollowing diseases : giant cell tumor(6), telangiectatic osteosarcoma(4), aneurysmal bone cyst(3), synovialsarcoma(3), chondroblastoma(2), soft tissue tuberculous abscess(2), hematoma(2), hemangioma(1), neurilemmoma(1),metastasis(1), malignant fibrous histiocytoma(1), bursitis(1), pyogenic abscess(1), and epidermoid inclusioncyst(1). Fourteen benign tumors and ten malignant, three abscesses, and two hematomas were included. Unlike truetumors, hematomas, tuberculous abscesses and the epidermoid inclusion cyst showed only one fluid-fluid level in aunilocular cyst. On T1-weighted images, the signal intensities of fluid varied, but on T2-weighted images,superior layers were in most cases more hyperintense than inferior layers. CONCLUSION: Because fluid-fluid layersare a nonspecific finding, it is difficult to specifically diagnose each disease according to the number offluid-fluid levels or signal intensity of fluid. In spite of the nonspecificity of fluid-fluid levels, they werefrequently seen in cases of giant cell tumor, telangiectatic osteosarcoma, aneurysmal bone cyst, and synovialsarcoma. Nontumorous diseases such as abscesses and hematomas also demonstrated this finding.