During extra corporeal photopheresis (ECP) 1.500 ml blood is extracted. Leucocytes are separated by centrifugation. Methoxsalen is added to the leucocytes and activated by UVA light. After treatment leucocytes are returned to the patient. ECP is a part of the treatment of acute and chronic graft-versus-host disease (GVHD) in bone marrow transplant patients and severe cutaneous T-cell lymphomas (CTLC). Studies have documented an effect of ECP in GVHD and CTLC. ECP has been used to treat other T-cell mediated diseases, the documentation however is sparse. The side effects of ECP are few and not serious.