The wild-type Wilms tumor gene, WT1, is overexpressed in myelodysplastic syndrome (MDS) as well as acute myeloid leukemia. In a phase I clinical trial of biweekly vaccination with HLA-A*2402-restricted WT1 peptide for these malignancies, 2 patients with MDS developed severe leukocytopenia in association with a reduction in leukemic blast cells and levels of WT1 messenger RNA (mRNA) after only a single vaccination with 0.3 mg of WT1 peptide. These results indicated that the WT1-specific cytotoxic T-lymphocytes (CTLs) elicited by WT1 vaccination eradicated the WT1-expressing transformed stem or progenitor cells and that MDS patients with little normal hematopoiesis required a new strategy of WT1 vaccination to avoid severe leukocytopenia. We describe the first trial for a 57-year-old male patient with chronic myelomonocytic leukemia who was vaccinated biweekly with a small quantity (5 microg/body) of WT1 peptide. After the start of vaccination, the leukocyte and monocyte counts (13,780/microL and 1930/microL, respectively) gradually decreased to within the normal range in association with a reduction in the WT1 mRNA level. Simultaneously, the percentage of WT1-specific CTLs as measured by the HLA-WT1 tetramer assay increased. This case demonstrates for the first time that vaccination with as little as 5 microg of WT1 peptide can induce WT1-specific immune responses and resultant clinical responses.