This is a phase 2 study of extracorporeal photopheresis (ECP) for treatment of chronic graft versus host disease following allogeneic hematopoietic stem cell transplantation. Extracorporeal exposure of human mononuclear cells to ultraviolet A radiation following photosensitization with 8-methoxypsoralen has proven to be an effective treatment for cutaneous T-cell lymphoma. Small, single institution studies have suggested efficacy in other T-cell mediated diseases such as solid organ rejection and graft versus host disease. This a phase 2 study of ECP for treatment of chronic graft versus host disease. Eligible patients must have objective evidence of chronic graft versus host disease that is refractory to conventional therapy or must have steroid-dependent disease that does not permit steroid dose reduction. The primary objective of this study is to better define the safety, efficacy and mechanism of action of ECP for treatment of patients with chronic graft versus host disease.