Magnetic resonance imaging (MRI) of the bone marrow was performed in 29 patients with leukemia, aplastic anemia, or lymphoma who were scheduled for bone marrow transplantation, and in 12 normals. T1-weighted coronal images (TR600/TE40) of the pelvis and proximal femurs demonstrated marrow pathology in adult patients. A simple MR grading system was developed to classify patterns of marrow involvement, and MR grading of cellularity was correlated with marrow histology. Normal marrow produced a relatively high signal intensity reflecting the predominance of short T1 fat in the marrow space. MRI of pretransplant patients with chronic myelogenous leukemia and acute leukemia in relapse demonstrated a markedly decreased marrow signal, consistent with the replacement of marrow fat by longer T1 neoplastic tissues. Aplastic anemia could not be differentiated from normal with the pulse sequences employed. Marrow involvement by Hodgkin's lymphoma was detected as diffuse marrow infiltration with superimposed focal areas of even lower signal intensity, reflecting the nodular nature of Hodgkin's. These results indicate that infiltrative marrow disorders can be sensitively detected by MRI.