Chronic graft-versus-host disease (cGVHD) is an important determinant of long-term morbidity and mortality in allogeneic stem cell transplantation patients. Because cGVHD has clinical, histologic, and laboratory findings of autoimmune diseases and anti-B-cell therapy has shown efficacy in autoimmune diseases, we hypothesized that monoclonal anti-CD20 antibody therapy might improve patients with cGVHD. We treated 5 men and 1 woman with therapy-refractory extensive cGVHD with anti-CD20 monoclonal antibody. Intravenous infusion was given at a weekly dose of 375 mg/m(2) for 4 weeks. In case of incomplete clinical response, additional courses of 4 weeks were given. Five patients responded to treatment with marked clinical, biochemical, and histologic improvement. One patient failed to respond. Anti-CD20 monoclonal antibody seems to be effective in cGVHD. A controlled trial is mandatory to confirm these results. The outcome of this study suggests a participating role of B cells in the pathogenesis of cGVHD.