Wilms' tumor gene 1 (WT1) gene expression was analyzed in 32 patient with acute myeloid leukemia (AML) and 18 with acute lymphoblastic leukemia (ALL) to investigate whether it could serve as a MRD marker. Ninety-four percent of patients with acute leukemia showed high WT1 expression at presentation. WT1 expression as a MRD marker was evaluated in 36 patients. The rise of WT1 expression preceded the hematological relapse by approximately 4 months (mean time 129 days; range 6-298). The prognostic significance of WT1 expression was analyzed in 30 patients with AML. WT1 expression higher than 20 WT1 copies /10(4)ABL copies after induction and consolidation chemotherapy was associated with shorter OS. WT1 expression analysis could be a useful tool for MRD monitoring in acute leukemia.