The Wilms tumor gene WT1 is expressed in leukemias and various kinds of solid tumors, including lung and breast cancer, and exerts an oncogenic function in these malignancies, suggesting that WT1 protein is a novel, overexpressed tumor antigen. The WT1 protein, in fact, is an attractive tumor rejection antigen in animal models. Stimulation in vitro of peripheral blood mononuclear cells with HLA-A*2402--and HLA-A*0201--restricted 9-mer WT1 peptides elicits WT1-specific cytotoxic T-lymphocytes (CTLs), and the CTLs kill endogenously WT1-expressing leukemia or solid tumor cells. Furthermore, WT1 immunoglobulin M (IgM) and IgG antibodies are detected in patients with hematopoietic malignancies such as acute myeloid leukemia, chronic myeloid leukemia, and myelodysplastic syndromes, indicating that WT1 protein overexpressed by leukemia cells is indeed immunogenic. Taken together, these results demonstrate that WT1 protein is a promising tumor antigen for cancer immunotherapy against leukemias and various kinds of solid tumors, including lung and breast cancer.