Growing understanding of the important role of B lymphocytes in alloreactivity has paved the way for evaluating anti-B cell therapy with rituximab in patients undergoing allogeneic hematopoietic cell transplantation. Data suggesting a beneficial reduction in incidence and severity of acute graft-versus-host disease (GVHD) are limited to non-randomized studies from single institutions using higher than conventional doses of rituximab. Additionally, rituximab is used as an effective treatment of corticosteroid-refractory chronic GVHD with good responses, particularly in cases of dermatologic and mucosal involvement. Post-transplant administration of rituximab appears to reduce the rate of chronic GVHD in preliminary studies.