WT1 has been identified as a new molecular marker in acute leukemias, although the significance of WT1 overexpression remains unclear. The purpose of the present study was to investigate WT1 expression levels in children with acute lymphoblastic and myeloid leukemia. Twenty-one children were enrolled in the study. Thirteen patients (6 males-7 females), 11 with ALL and 2 with AML and a mean age of 8 years old (range from 4 months to 15 years old), were studied. Normal bone marrow from 8 children with non malignant hematological diseases was used as control. Total RNA was isolated either from frozen bone marrow samples using the TRIzol reagent protocol as described by the supplier or from mononuclear cells fractioned by Fycoll-Isopaque density gradient centrifugation, using the QIAamp RNA mini protocol following the manufacturer’s instructions. Qualitative PCR reactions were performed according to the protocols established by the European BIOMED 1 Concerted Action. Five of the 13 patients studied (38.5%), expressed high levels of WT1. Specifically, five of the 13 patients, three with ALL and two with AML, were positive for WT1. Four of the WT1 positive patients, who had additional adverse prognostic factors according to their treatment protocols have succumbed to their disease. One of the patients of the control group, while remaining under investigation for thrombocytopenia and leukopenia, revealed significantly high WT1 expression levels and was therefore excluded from the control group. In conclusion, though it has been reported that leukemic patients express WT1 at a higher percentage than the children in this study, our above findings are concomitant with the fact that WT1 is considered to be an adverse prognostic factor.