Background: The B-lymphocyte stimulator (BLYS) protein is known to regulate immunoglobulin in normal B cells, and be overexpressed in B-cell malignancies, including Waldenstrom macroglobulinemia (WM). Patients and Methods: This trial evaluated the safety and activity of belimumab, a monoclonal antibody targeting BLYS, in 12 patients with WM in a single-arm phase II study. Results: Ten patients had stable disease with therapy, although no objective responses were seen. Correlative studies showed patients to have low or undetectable baseline serum levels of BLYS, with the administration of belimumab having no effect on B-cell numbers. Conclusion: Belimumab cannot be recommended as a single-agent therapy for the treatment of symptomatic WM, although further evaluation in combination with other agents would be justified. ? 2013 Elsevier Inc. All rights reserved.