Graft-versus-host disease (GvHD) is a common and often serious complication of hematopoietic stem cell transplantation. There are two major forms of GvHD: an acute form which develops in the first 100 days after HSCT, and a chronic form which develops later. Chronic GvHD is a multiorgan syndrome with many features of autoimmune diseases, such as sclerodermatous skin changes, cholestasis, pulmonary fibrosis, xerostomia, oral ulcerations, myositis and fasciitis. Unlike acute GvHD which is characterized by acute alloreactivity, the etiology of chronic GvHD is controversial and is believed to be either an extension of acute GvHD and/or a result of dysfunctional immune reconstitution with generation of autoantibodies and autoreactive T-cell clones. GvHD is usually treated with corticosteroids and other immunosuppressants which do not always succeed in arresting its evolution. Extracorporeal photochemotherapy has been used in patients with both acute and chronic GvHD.