Some 21 years have elapsed since the first use of artificial radioactive isotopes in cancer diagnosis and treatment. Enthusiasts were initially hopeful that in the near future the profession might witness outstanding examples of selective localization of radioactive materials in neoplastic tissue, making it easy to detect such lesions and possibly providing a means to deliver tumor doses much higher than would be received by the adjacent normal tissues. However, up to the present time, only two radioactive isotopes have been developed to provide important therapeutic applications through relatively selective localization— radiophosphorus (P32) in polycythemia vera and radioiodine (I131) in a small percentage of thyroid carcinomas. This fact has confirmed the belief of many that the greatest contribution of radioactive isotopes in the field of cancer lies in tracer applications. Nevertheless, isotopes do have a place in diagnosis and therapy. The following remarks will attempt to summarize our