The Wilms' tumor gene WT1 is highly expressed in leukemias and myelodysplastic syndrome (MDS), and WT1 expression levels increase along with the disease progression in chronic myeloid leukemia and MDS. We previously reported that IgM and IgG WT1 antibodies were detected with significantly higher detection rate and antibody titers in leukemias and MDS compared to those in healthy volunteers. In this study, whether IgG humoral immune responses against WT1 protein were Th1- or Th2-type were determined by measurement of four subclasses of IgG WT1 antibody, IgG1, IgG2, IgG3, and IgG4. In leukemias and MDS, Th1-type WT1 antibodies such as IgG1, IgG2, and IgG3 were significantly increased in both detection rate and antibody titers compared to those in healthy volunteers, whereas Th2-type WT1 antibody such as IgG4 did not increase. These results showed that Th1-biased humoral immune responses against WT1 protein were generated in leukemias and MDS. These results should allow us to consider that Th1-biased cellular immune responses against WT1 protein, which was essentially needed for cancer immunotherapy targeting WT1, should be elicited in patients with hematopoietic malignancies.