The Wilms tumor gene was identified as a tumor suppressor gene responsible for a particular type of kidney tumor. Several years ago, it was demonstrated that it is also overexpressed in acute and chronic leukemias. Although the exact role of this gene in the hematopoietic system is still quite completely obscure, it represents a reliable marker for the detection of the presence of leukemic cells. WT1 quantitative assessment may therefore represent a useful tool for the diagnosis and follow up of leukemia patients. In this chapter, we describe the method for the quantification of WT1 transcript by real-time polymerase chain reaction.