Extracorporeal photochemotherapy (ECP) has been associated with clinical improvement in several patients with acute and chronic graft-versus-host disease (cGVHD) after allogeneic bone marrow transplantation, but the mechanism of action is unknown. This study tested the hypothesis that in patients with cGVHD, ECP modulates alloreactivity by affecting activated lymphocyte populations or by altering the interaction between effector lymphocytes and antigen-presenting cells (APCs). Ten patients who had refractory cGVHD were treated with ECP, and the clinical response to and immunologic effects of this therapy were assessed. Seven patients had a response and 3 had no change in clinical manifestations of cGVHD. One patient died from catheter-related sepsis. Immunologic effects observed after ECP included normalization of inverted ratios of CD4 to CD8 cells, an increase in the number of CD3-CD56+ natural killer (NK) cells, and a decrease in CD80+ and CD123+ circulating dendritic cells. The results suggest that ECP modulates both NK cells and APC populations in patients with cGVHD.