In 35 patients with malignant bone or bone marrow disease, comparative BMS and MRT were performed. Both methods are useful for the early recognition of malignant changes in the skeleton, frequently at a stage when radiographs and bone scans are still negative. BMS provides a global picture of the marrow and can demonstrate focal or diffuse defects within malignant infiltration. Portions of the skeleton which do not contain erythropoietic marrow, or those obscured by high liver or spleen activity, cannot be evaluated. MRT shows the highest sensitivity. The lesions are usually characterised by circumscribed low signal (T1) or high signal (T2) areas. The size and extent were accurately demonstrated. Definite information regarding malignancy was not possible by MRT alone. The method is limited because it is slow, expensive and not easily available. MRT should therefore be reserved for specific problems and is not suitable for examination of the entire bone marrow as, for instance, for staging.