Vitamin K2 is reported to induce apoptosis or differentiation of leukemic cell lines in vitro. We administered a vitamin K2 analog, menatetrenone, at 45 mg daily to 23 patients with myelodysplastic syndrome (MDS): 13 patients with RA, 2 with RARS, 6 with RAEB and 2 with RAEB-T. Good response (GR) and partial response (PR) were defined as an increase of hemoglobin concentration exceeding 2 g/dl and 1-2 g/dl without transfusion, respectively. Six of the RA patients showed improvement of anemia (GR, 3 patients; PR, 3 patients). RA patients who did not have a hypocellular bone marrow and were transfusion-independent tended to be responsive to vitamin K2 therapy in combination with vitamin D3 or anabolic steroids. No adverse effect of vitamin K2 was observed, and the time required to obtain the hematological response was short, being 3 months on average. We believe that vitamin K2 therapy has potential as a treatment for patients with MDS.