The efficacy and toxicity of treatment with 1400 +/- 100 MBq of Re-186-HEDP were evaluated in women with osseous metastatic breast cancer. The follow-up period was fourteen weeks. The efficacy of treatment was assessed by a) a pain and performance questionnaire that patients were asked to complete daily and b) a CT scan comparison of a randomly preselected osseous lesion before and 30 weeks after Re-186-HEDP i.v. application. The response to treatment was also evaluated by using the Kamofsky Index. Two out of fourteen women (14%) experienced loss of pain, 6 experienced obvious and 2 some improvement. No change was observed in 4 patients. Five patients manifested a flare response to treatment, with increase in pain within the first, 4 to 5 days after Re-186-HEDP administration. Five patients showed a decrease in platelet levels and absolute number of polymorphonuclear blood transfusion; no neurologic side effects were observed. Re-186-HEDP appears to be a useful new radiopharmaceutical for pain palliation induced by osseous metastases due to breast cancer. Compared to Sr-89 chloride efficacy, it provides longer-lasting analgesia, and when needed it can be reinjected with less risk due to its improved physico- and radiochemical properties.