Radioactive isotopes have been increasingly used for medical diagnosis since the late 1940s. The number in common use is surprisingly limited with a few, like99rnTc1being used to label several different pharmaceuticals that concentrate in specific organs. Imaging of such organs is probably now the main diagnostic clinical use of radioisotopes but other tests such as bone mass measurement, immunoassay, blood volume determination, neutron activation analysis and whole body radioactivity measurement also require them. Most tests can be satisfactorily performed with a radiation dose to the patient of only about 10 mGy. Transmission computerized tomography and ultrasonic imaging made an initial impact on the diagnostic use of radioisotopes, but since the latter measure and make a visual display of organ function rather than anatomical structure, their use is likely to continue growing.