A 54-year-old man showed evidence of disease progression and a reduction in donor chimaerism by molecular microsatellite analysis 6 months after an allogeneic peripheral blood stem cell transplant for chronic idiopathic myelofibrosis. He was treated with a single infusion of donor leucocyte infusions (DLI), which led to the development of mild acute graft versus host disease (GVHD) and the rapid restoration of full donor haemopoiesis. This subsequently led to a progressive reduction in marrow fibrosis from grade IV to grade I over the following 6 months. We believe that this is the first report to provide clear evidence for the efficacy of DLI in this setting, which also provides evidence for the existence of a T-cell-mediated 'graft vs. myelofibrosis' effect similar to that seen against other haematological malignancies.