MR is a highly sensitive alternative to plain films, CT, and radionuclide studies for the imaging of normal and abnormal marrow and can characterize differences between fatty, fibrotic, cellular, hypercellular, and hemosiderotic marrow. MR is helpful in depicting the extent of disease and has been a useful method to follow the clinical course of many disorders. It has been found to be particularly useful in explaining the unrepresentative biopsy, as the distribution of many diseases is frequently heterogeneous as exemplified by the mixed fatty and cellular patterns of aplastic anemia, myeloma, lymphoma, and skeletal metastases. Patterns of cellular and fatty marrow in the epiphysis and apophysis after marrow reconversion were not completely understood prior to the introduction of MR scanning. Because it has the advantage of imaging the entire bone marrow compartment (unlike the situation with biopsy on aspiration), MR allows a better understanding of the distribution of skeletal disease.