Aims: Despite the clinical importance of the leukemic transformation of chronic myeloproliferative neoplasms (MPNs), very little is known about markers that predict leukemic transformation. We studied WT1 expression in 37 MPN patients diagnosed as bcr-abl negative and JAK2 (V617F) positive with a molecular genetic test, and 23 healthy controls. Results: WT1 expression is higher in MPN patients compared with normal controls (p=0.002). According to the WT1 expression levels, patients were divided into two groups: high (>= 0.205) and low (0-0.205) WT1 expression. Two out of six patients with a high WT1 expression level transformed to myelodysplastic syndrome at a 42- and 46-month follow-up, respectively. Conclusions: Our results suggest that the overexpression of WT1 may play an important role in the leukemic transformation of MPNs.