Prostate carcinoma is the most commonly associated with osseous metastases malignancy in males. The lesions, being usually of a mixed sclerotic/lytic variety and less often of the pure sclerotic type, need to be treated by a bone seeking radioactive element with an as low as possible radiobiological burden on the surrounding (peritumoral) tissues. Rhenium-186-HEDP was used to treat these osseous metastatic lesions due to its bone seeking kinetics attractive radiochemical properties. Of a total of 16 prostate cancer patients. 3 experiment loss of pain, 8 experienced obvious and 2 some improvement. No change was observed in 3 patients. Ten patients manifested a flare syndrome increasing pain approximately 2 to 6 days, after Re-186-HEDP i.v. application. Six patients showed a definite and 9 a slight decrease in platelet levels and absolute number of polymorphonuclear white blood cells, up to fourth week following treatment. One patient underwent a whole blood transfusion and in 2 peripheral neuropathy was observed lasting about 9 to 12 days. Re-186-HEDP appears to be a promising new metal ion complex for the palliation of painful bone metastases in prostate cancer. Compared to Sr-89 therapy, it shows a longer analgetic efficacy and has the advantage of emitting gamma rays, a fact which facilitates dosimetric calculations.