Ultraviolet irradiation is known to diminish the functional capacity of cells of the immune system. We have used ultraviolet A irradiation in combination with psoralen (PUVA) to treat three patients with drug-resistant graft-versus-host disease (GVHD) of the skin or mouth. Each of the three patients had received an HLA-identical sibling bone marrow transplant for haematological malignancy or severe aplastic anaemia. One patient developed acute GVHD of skin and mouth and two patients developed chronic GVHD of the mouth, which did not respond to conventional immunosuppressive drugs including cyclosporin and methyl prednisolone (all three patients) and anti-thymocyte globulin (two patients). PUVA irradiation to the skin was given at a dose of 0.75-1.25 J (joules) daily for four days per week and that to the mouth at 0.75 J daily four days per week. Before each treatment, 8-methoxypsoralen (0.6 mg/kg) was given orally as a photosensitizer. Each patient improved considerably and in each case the dosage of conventional immunosuppression was reduced. No flare was noted in the two patients with chronic GVHD of the mouth at 1 month after the end of treatment. PUVA irradiation is a useful therapeutic adjunct in GVHD affecting skin and mouth and appears to confer a steroid or cyclosporin sparing effect.