Graft-versus-host disease (GVHD) causes significant morbidity and mortality in patients undergoing allogeneic bone marrow transplantation following either a conventional or reduced-intensity preparative regimen. In a murine model, inactivation of host dendritic cells (DCs) was associated with a significant reduction in acute GVHD, suggesting that host DCs may play an important role in the pathogenesis of acute GVHD. The role of host DCs in the development of GVHD following allogeneic stem cell transplantation in humans, however, is unclear. We examined DC chimerism in patients with various hematologic malignancies who underwent a reduced-intensity preparative regimen of extracorporeal photophoresis, pentostatin, and reduced-dose total body irradiation (n = 21) or a conventional preparative regimen of cyclophosphamide and total body irradiation (n = 3). Full donor hematopoietic reconstitution was demonstrated in 19 of 21 patients who underwent a reduced-intensity preparative regimen and in all patients who underwent a conventional preparative regimen. Grade 0 to I acute GVHD and limited or no chronic GVHD were observed in 18 patients who underwent a reduced-intensity regimen and 1 patient who underwent a conventional regimen who achieved full donor DC chimerism at day +100 posttransplantation. In contrast, grade II to IV acute GVHD and extensive chronic GVHD were observed in the 2 patients who underwent a conventional regimen and the 1 patient who underwent a reduced-intensity regimen who had host rather than donor DC chimerism. The persistence of host DCs at day +100 posttransplantation is correlated with the development of severe acute and chronic GVHD (P =.001). Host DCs may represent a therapeutic target for reducing GVHD in allogeneic bone marrow transplants.