Wilms tumor 1 (WT1) is considered to be a promising target of cancer treatment because it has been reported to be frequently expressed at high levels in various malignancies. Although WT1-targeted cancer treatment has been initiated, conclusive detection methods for WT1 are not established. The present study aimed to consolidate immunohistochemistry for WT1 with statistical basis. Transfected cells with forced WT1 expression yielded specific western blot bands and nuclear immunostaining; cytoplasmic immunostaining was not specifically recognized. Immunohistochemistry, western blotting, and quantitative reverse transcriptase-polymerase chain reaction were performed in 35 human cell lines using multiple WT1 antibodies and their results were quantified. Relationships among the quantified results were statistically analyzed; the nuclear immunostaining positively correlated with western blot bands and mRNA expression levels, whereas cytoplasmic immunostaining did not. These results indicate that nuclear immunostaining reflects WT1 expression but cytoplasmic immunostaining does not. The nuclear immunostaining was barely (3/541) observed in primary cancer of esophagus, bile duct, pancreas and lung. Although the present study has some limitations, the results indicate that the cytoplasmic immunostaining does not correlate with actual WT1 expression and prompts researchers to carefully evaluate target molecule expression in treatment of cancer.