Multiple myeloma (MM) remains an incurable illness affecting nearly 20,000 individuals in the United States per year. High-dose melphalan (HDM) with autologous hematopoietic stem cell support (ASCT) is one of the mainstays of therapy for younger patients, but little advancement has been made with regards to conditioning regimens. We opted to combine 153Samarium ethylenediaminetetramethylenephosphonate (153Sm-EDTMP), a radiopharmaceutical approved for the palliation of pain caused by metastatic bone lesions, with HDM and ASCT in a Phase II study. Individualized doses of 153Sm were based on dosimetry and were calculated to deliver 40 Gy to the bone marrow. The therapeutic dose of 153Sm-EDTMP was followed by HDM and ASCT. Forty-six patients with newly diagnosed or relapsed disease were treated. Study patients were compared to 102 patients contemporaneously treated with HDM and ASCT. Fifty-nine percent of study patients achieved a very good partial response (VGPR) or better. With a median follow-up of 7.1 years, the median overall survival and progression free survival (PFS) from study registration was 6.2 years (95% CI 4.6–7.5 years) and 1.5 years (1.1–2.2 years), respectively, which compared favorably to contemporaneously treated non-study patients. Addition of high-dose 153Sm-EDTMP to melphalan conditioning appears to be safe, well tolerated, and worthy of further study in the context of novel agents and in the Phase III setting. Am. J. Hematol. 2010. © 2010 Wiley-Liss, Inc.