The value of magnetic resonance imaging (MRI) and computed tomography (CT) in the diagnosis of bone tumours was investigated in a prospective study of 25 patients. MRI is superior to CT because it permits multidirectional exposures. Moreover, the tumour can be readily distinguished from the neurovascular structures without injection of contrast medium. MRI gives better contrast than CT, making it possible to study the relationship to the soft tissues, bone marrow and joints more accurately. On the other hand, CT gives a better picture of the destruction of cortical bone. With neither MRI nor CT can the exact tumour length be measured. Neither MRI nor CT permits an exact, reliable diagnosis. Owing to the relatively slow exposure technique in combination with respiratory movements, depiction of the thoracic wall is less satisfactory with MRI than with CT. If both techniques are available, MRI is preferred. In view of the fact that MRI apparatus is still less widely available, it should be borne in mind that CT also allows an adequate investigation of skeletal lesions.