The occurrence of acute or chronic graft-versus-host disease (GVHD) and its prevention/treatment induce further immunologic compromise. Graft-versus-host disease (GvHD) accounts for significant morbidity and mortality in recipients of allogeneic bone marrow transplants (BMT) or peripheral blood stem cell (PBSC) transplants. The condition occurs as donor lymphocytes recognize the recipient's tissue as foreign and mount an inflammatory and destructive response in the recipient. GvHD has a predilection for epithelial tissues, especially skin, liver, and the mucosa of the gastrointestinal tract. This can lead to severe skin lesions, gastrointestinal hemorrhage, and liver failure. In addition, people with GvHD are especially susceptible to infection. Approximately 30-70% of people undergoing allogeneic HSCT for hematologic malignancies can anticipate experiencing acute and/or chronic GVHD. The physical consequences depend on the severity of GVHD, but moderate to severe organ system involvement is associated with substantial medical morbidity and mortality. The current standard of care treatment for patients with chronic graft-vs-host disease is steroid treatment with or without a calcineurin inhibitor. This treatment, while effective for some, does not control the chronic gvhd of all. The use of extracorporeal photopheresis (ECP) was developed in patients with several inflammatory and autoimmune diseases, including scleroderma and rheumatoid arthritis. Successful use of ECP has also been reported in patients with cutaneous T cell lymphoma, the Sezary syndrome variant, cardiac and lung transplant rejection and other T cell mediated/autoimmune and autoimmune diseases and even after facial transplantation. More recently, ECP has proven to be a highly effective useful tool in the treatment of GvHD.