This phase 1 study (clinical trial NCT00477815) was conducted to determine the maximum tolerated dose (MTD) of yttrium-90 ibritumomab tiuxetan (90Y-Zevalin) with high dose melphalan (HDM) therapy in multiple myeloma (MM) patients undergoing autologous stem cell transplantation (ASCT). In a 3+3 trial design, 30 patients received rituximab 250 mg/m2 with indium-111 ibritumomab tiuxetan (111In-Zevalin) for dosimetry (day -22); rituximab 250 mg/m2 with escalating doses of 90Y-Zevalin (day -14); melphalan 100 mg/m2 (days -2, -1) followed by ASCT (day 0) and sargramostim (GM-CSF, day 0) until neutrophil engraftment. Each patient's 111In-Zevalin dosimetry data were used to calculate the dose of 90Y-Zevalin (in mCi) to deliver 10, 12, 14, 16, 18 or 20 Gy to the liver. Dose limiting toxicities were seen in 3 patients. The overall response rate was 73% (22/30) with stringent complete response in 2 patients; complete response, 5; very good partial response, 12; and partial response, 3. The median PFS was 16.5 months and the median overall survival was 63.4 months. In MM, the MTD of 90Y-Zevalin with HDM is 18 Gy to the liver. The addition of radiation with novel delivery methods such as radioimmunotherapy combined with standard transplant regimens warrants further study.