BACKGROUND Graft-versus-host disease (GVHD) remains a cause of long-term morbidity after allogeneic hematopoietic stem cell transplantation, and recent studies indicate that extracorporeal photophoresis (ECP) is useful for treatment of steroid-refractory GVHD although the mechanisms are unclear. Antigen-presenting cells (APCs) such as dendritic cells have a central role in GVHD, and apoptosis of APCs by HLA-DR monoclonal antibody (MoAb) has been documented in vitro and in vivo. Monocytes have been identified as precursors of dendritic cells in vivo and particularly under conditions of inflammation. STUDY DESIGN AND METHODS This study examined whether ECP altered the survival of peripheral blood monocytes from patients with GVHD, monocyte apoptosis after engagement of HLA-DR antigens with MoAb, and monocyte apoptosis after allointeraction with primary CD4+ T lymphocytes. Samples from patients from two centers were studied. RESULTS It is reported here that ECP induced apoptosis of monocytes over a period of at least 48 hours. ECP also clearly increased cell death of monocytes after engagement of HLA-DR antigens with MoAb. In contrast, engagement of HLA-DR by allointeraction failed to induce significant cell death of monocytes, and this was unaltered by ECP treatment. CONCLUSION These data reveal that monocytes from patients with GVHD are sensitive to HLA-DR-mediated apoptosis and that ECP treatment increases sensitivity to both spontaneous and HLA-DR-mediated apoptosis. Therefore, ECP treatment in combination with HLA-DR MoAbs could rapidly deplete monocytes and thereby reduce the contribution of monocyte-derived dendritic cells to GVHD.