Tin-117 has certain physical characteristics (half-life of 13.6 days, low-energy-conversion electrons, gamma photon of 158.6 keV) that suggest that it may be a favorable agent for radionuclide therapy. It has been shown in animal models that Sn-117m in the chemical form Sn(4+)diethylenetriaminepentaacetic acid localizes selectively in bone. The authors therefore studied its whole-body distribution in 10 patients to obtain absorbed dose estimates for therapy. The results showed that more than 50% of the administered activity was absorbed in the bones of patients with metastatic carcinoma. Retention was determined primarily by radioactive decay. For adult men, the radiation absorbed dose estimate averaged 54.8 mGy/MBq (203 rad/mCi) to bone surfaces and 6.1 mGy/MBq (22.6 rad/mCi) to the red marrow. All other tissues received less than 1/10 of the dose received by red marrow. These results suggest that a clinical therapeutic trial should be attempted.