Abstract: The invention is a method for detecting the presence of or a predisposition to cancer in mammalian individuals. A trace element, specifically tin, is administered to the individual in the form of a radioactive isotope or a compound containing a radioactive tracer. The tin-bearing compound thereafter accumulates in the various bodily organs and is excreted. Indicative of the presence of cancer or a predisposition is the lack of accumulation of the radioactive tin compound in the thymus and lymphatic system of the mammal. By comparison of the radioisotope concentration in the urine and blood, the early detection of a precancerous or leukemic condition can be made. The method is applicable to a wide variety of mammals, including human beings.