Wilm's tumor gene WT1, which has an oncogenic function, is expressed in various kinds of hematological malignancies and solid cancers. WT1 antibodies at higher titers and WT1-specific cytotoxic T lymphocytes (CTLs) at higher frequencies were detected in cancer patients than in healthy donors, indicating that WT1 protein was immunogenic. Furthermore, WT1-specific immune responses are considered to be involved with Graft versus Leukemia effect in the context of hematopoietic stem cell transplantation. These findings provided us with the rationale for cancer immunotherapy targeting WT1. Clinical trials of WT1 peptide vaccination for cancer patients were started, and WT1 vaccination-driven immunological responses and clinical responses, including reduction of leukemic cells, reduction of M-protein amount in myeloma, and shrinkage of solid cancer, were observed. Further enhancement of efficacy of WT1 peptide vaccine can be expected by co-administration of WT1-specific helper peptide or anti-cancer chemotherapy agent. WT1 peptide vaccination in the setting of MRD (minimal residual disease) may prolong "progression-free survival time", or decrease "relapse rate".