Rhenium-188-HEDP (¹⁸⁸Re-HEDP) is a new and attractive radiopharmaceutical for the treatment of metastatic bone pain. As a product of ¹⁸⁸W/¹⁸⁸Re generator, it is convenient for clinical therapeutic use with a short physical half-life of 16.9 h and a maximal β-energy of 2.1 MeV. We investigated the effect of ¹⁸⁸Re-HEDP on pain relief, analgesic intake and impairment of bone marrow function in 27 patients with bone metastases induced from prostate cancer. All patients were interviewed using a standardised set of questions before, and after therapy for 12 weeks. The patients were treated with 2700-3459 MBq of ¹⁸⁸Re-HEDP. Blood samples were taken weekly for 12 weeks, and a blood count was performed. Patients described an improvement on the Karnofsky performance scale from 74±7 to 85±9% 12 weeks after therapy (P=0.001). The pain score showed a maximum decrease from 44±18 to 27±20% in the third to the eight week after therapy (P=0.009). Seventy-six percent of the patients described a pain relief without increase of analgesic intake. Twenty percent of the patients could discontinue their analgesics and were pain free. Mean platelet count decreased from (286±75)*10³ μl ^-1 to (215±92)*10³ μl^-1, and mean leucocyte count from (7.7±1.5)*10³ μl ^-1 to (6.0±1.9)*10³ μl^-1 in the second to the fourth week after therapy. The maximal differences between the values of platelets and leucocytes before and after therapy were not statistically significant (P=0.021 and 0.094). In conclusion, ¹⁸⁸Re-HEDP is an effective radiopharmaceutical used in the palliative treatment of metastatic bone pain in prostate cancer and shows minimal bone marrow toxicity.