Wilms' tumor gene 1 (WT1) is gaining increasing attention as a therapeutic target molecule due to its common expression in acute leukemias and its involvement in cell proliferation. Here, we reported on WT1 messenger RNA expression levels at diagnosis in a series of 238 adult acute lymphoblastic leukemia (ALL) samples of various subtypes and clinical outcome. WT1 expression was found in 219 out of 238 ALL samples (92%). Compared to a cohort of acute myeloid leukemia patients, the median WT1 expression level in ALL was significantly lower with large variations among different ALL subgroups. Specifically, WT1 expression levels were low in mature B-ALL and highest in ALL cases with co-expression of myeloid markers, making it a useful therapeutic target molecule in adult ALL with the exception of mature B-ALL. Cox regression analysis, considering ALL phenotype as well as molecular-cytogenetic subsets, revealed no independent prognostic role of WT1 expression level for disease-free and overall survival.