Ten years after being diagnosed with polycythemia vera, a 55-year-old woman required frequent blood transfusion due to secondary myelofibrosis. She underwent reduced-intensity stem cell transplantation (RIST) from an HLA-identical sibling donor. Since mixed chimerae were identified in the peripheral blood at day 35, cyclosporine was withdrawn. At day 73, she developed acute graft-versus-host disease of the liver, while simultaneous resolution of splenomegaly occurred and complete donor chimerism in the peripheral blood was achieved. Frequent red blood cell transfusion was required until day 300 after transplantation. Thus, RIST for an older patient with secondary myelofibrosis was successful without severe treatment-related morbidity. This case suggests that RIST could be an effective treatment modality for secondary myelofibrosis.