Expression of the Wilms' tumor gene WT1 was examined in 59 cases of colorectal adenocarcinoma to examine the involvement of WT1 in tumorigenesis. Quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR) showed that WT1 mRNA was expressed in the range from 7.2 x 10(-5) to 4.9 x 10(-1) levels (WT1 expression level in K562 leukemic cells was defined as 1.0) in all (100%) of the 28 cases of colorectal adenocarcinoma examined, and that the WT1 mRNA expression levels were higher in 20 (71%) of the 28 cases compared to those of normal-appearing mucosal tissues examined. Immunohistochemical analysis using an anti-WT1 antibody was performed on 46 cases of colorectal adenocarcinoma (15 of the 28 cases with WT1 mRNA expression and 31 newly collected cases), and the expression of WT1 protein was detected in 41 (89%) of the 46 cases. The direct sequencing analysis of the WT1 genomic DNA showed no mutations in any of the 10 exons of the WT1 gene in any of 5 different colorectal adenocarcinomas. These results may indicate an important role of the wild-type WT1 gene in tumorigenesis of colorectal adenocarcinoma.