Allogeneic stem cell transplantation represents the only available long-term control and possible cure of a number of hematological malignancies. A major obstacle to this treatment is the development of graft-versus-host disease (GVHD), affecting the majority of transplanted patients to some extent. Today, combinations of various cytotoxic and immunosuppressive drugs are used to prevent and treat GVHD, but many of them are associated with severe side-effects. Extracorporeal photophoresis (ECP) offers an alternative to chemo- and immunosuppressive therapy and confers apparently only mild side effects. The postulated rationale of ECP is to treat the patient's white blood cells ex vivo with ultraviolet irradiation after sensitization with 8-methoxypsoralen to dampen their immunoactivity. After engraftment the intervention group receives 2 consecutive ECP every week in 2 weeks then 1 ECP every week in 4 weeks ( a total of 8 ECP procedures).