The First International Symposium on Photopheresis in Hematopoietic Stem Cell Transplantation was held in Vienna, Austria with an educational grant from Therakos Inc. from 25 May to 27 May 2005. Three general issues were addressed: (1) pathophysiology of graft-versus-host disease (GvHD), (2) induction of immune tolerance and the immunology of phototherapy and (3) current standard treatment and prevention strategies of acute and chronic GvHD and the use of extracorporeal photopheresis (ECP). The objectives of the meeting were to open a dialogue among leading researchers in photobiology, immunology, and hematopoietic stem cell transplantation; foster discussions and suggestions for future studies of the mechanism of action of ECP in acute and chronic GvHD; and promote collaboration between basic scientists and clinicians. As can be seen from the summaries of the individual presentations, important advances have been made in our understanding of GvHD, including the use of photoimmunology interventions and the development of robust model systems. It is our expectation that data from photoimmunology studies can be used to generate hypotheses in animal models that can further define the mechanism of action of ECP and help translate the findings to clinical trials of ECP for the prophylaxis and treatment of both chronic and acute GvHD.