A 21-year-old man who had an increased number of eosinophils with morphological abnormalities, bone marrow fibrosis and multiple organ dysfunction failed to respond to methylprednisolone and hydroxyurea. He was diagnosed with hypereosinophilic syndrome (HES) probably due to myeloproliferative disorder, and underwent allogeneic bone marrow transplantation (allo-BMT) from an HLA-identical sibling. The engraftment was confirmed on day 21 after BMT, but the patient developed acute graft-versus-host disease (GVHD) with grade I veno-occlusive disease, and transient increase of eosinophils of the donor type followed by chronic GVHD of the extensive type. These complications were eventually controlled with cyclosporin A. The patient survived free of disease for more than a year after BMT. Allo-BMT seems to be a possible treatment of HES/MPD.