Wilms' tumor (WT) is a pediatric malignancy that occurs in embryonic kidney. Recently, a putative Wilms' tumor gene (WT1), located on chromosome 11p13, was isolated and characterized. We found constitutive expression of WT1 mRNA in eight out of 22 hematopoietic cell lines and seven out of 26 clinical samples which were derived from patients with various types of hematologic malignancies. WT1 mRNA was detected in four out of six myeloid cell lines, four out of 10 cases of acute myelocytic leukemia, three out of 15 lymphoid cell lines, one out of nine cases of lymphoid malignancies, and one out of six cases of chronic myelocytic leukemia in accelerated phase and blast crisis. One unclassified hematopoietic cell line and a case of myelodysplastic syndrome also expressed WT1 mRNA. No mutations were detectable in the cell lines by Southern blot analysis and a polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analysis in the four zinc finger domains of the WT1 gene. These results suggest that WT1 gene is expressed in several types of immature lymphoid or myeloid leukemia cells possibly without alterations of the WT1 gene.