Narrowband ultraviolet B phototherapy (NB-UVB) is a therapeutic alternative for haematopoietic stem cell transplantation-related skin graft-versus-host disease (GVHD). The beneficial effects of this intervention may be induced by direct irradiation of inflammatory cells in the skin; however, the putative involvement of indirect effects on systemic immunity has not been elucidated. To address this issue, 11 acute skin GVHD patients refractory to standard corticosteroid treatment and with no gut/liver involvement were treated with NB-UVB irradiation. The median number of treatments was 10 times, with a mean cumulative exposure of 6.36 J/cm(2). No other immunosuppressive therapy was initiated during irradiation. Eight patients achieved an objective complete response, two had a partial response, and one showed no change. None of the patients experienced progressive skin GVHD or newly diagnosed gut/liver GVHD. NB-UVB was well tolerated, with no patients discontinuing irradiation due to toxicity. We additionally demonstrated by flow cytometry that NB-UVB irradiation induces the increment of the proportion of regulatory T cell (Tregs) in patients' peripheral blood. These results suggest that NB-UVB may exert beneficial effects on steroid-refractory skin GVHD through the expansion of Tregs.