A total of 20 patients enrolled in a multicenter phase II dose escalation study of radioimmunotherapy (RIT) using yttrium-90-ibritumomab tiuxetan at two dose levels (22 and 30 MBq/kg) in 10 patients, combined with reduced intensity conditioning (RIC) using fludarabine, melphalan and alemtuzumab followed by allogeneic hematopoietic cell transplantation (HCT) from either matched-related (n=5) or matched-unrelated donors (n=15). Postgrafting immunosuppression with cyclosporine was administered. Diagnoses were diffuse large B-cell lymphoma (n=13), transformed CLL (n=4), blastic mantle cell lymphoma (n=2) and follicular lymphoma grade 3 (n=1). Median age was 51 (range, 29-69) years. All patients were high risk with relapsed/refractory disease or relapse after preceding autologous HCT. Median follow-up of patients alive was 1115 (range, 1006-1252) days. No directly RIT-related toxicities were observed. The cumulative incidence of non-relapse mortality was 30%. Incidences of grade II-IV acute and chronic GvHD was 45% and 70%, respectively. Kaplan-Meier estimated 3-year OS and EFS were 20% for both dose levels. In conclusion, dose escalation of RIT and combined use with RIC is feasible with no additional toxicity due to dose escalation. This study is registered on http://clinicaltrials.gov as NCT00302757.