▴ Samarium153Sm lexidronam is a medium energy beta-emitting radioisotope chelated to the tetraphosphate, ethylenediaminetetramethylenephosphonic acid (EDTMP).153Sm has a physical half-life of 1.9 days.▴ Samarium153Sm lexidronam concentrates in bone tissue and has a metastatic lesion:normal bone ratio of 4.▴ Clearance of nonskeletal radioactivity after administration of samarium153Sm lexidronam is rapid, and is almost complete from the blood and urine within 1 and 6 hours, respectively.▴ Some degree of pain relief was achieved in 72% of patients with bone metastases after a single dose of samarium153Sm lexidronam 1 mCi/kg (37 MBq/kg) compared with 44% of placebo recipients (p < 0.025). Relief was generally attained within 2 weeks and lasted for a median of 8 to 15 weeks.▴ After recurrence of pain, a second dose of samarium153Sm lexidronam may produce further pain relief in some patients.▴ The dose-limiting toxicity of samarium153Sm lexidronam is reversible myelosuppression.