The Wilms' tumor gene (wt-1) is expressed in the developing fetal kidney, gonads and in Wilms' tumors. Recently, the expression of wt-1 in leukemia-derived cell lines and cases of acute leukemias (AL) was reported. The present study was designed to investigate the potential of wt-1 as genetic marker for acute myelocytic leukemias (AML). Blast cells from 52 patients with AML, 14 patients in complete remission (CR) and four leukemic cell lines were examined for expression of wt-1 mRNA. Peripheral blood mononuclear cells (PBMNC) and bone marrow (BM) from 13 healthy persons were used as negative controls. RNA of the wt-1 gene was expressed in 41/52 (79%) patients with previously untreated AML. The majority of the 14 patients studied in CR lost wt-1 expression. In three out of the four patients in CR reappearance of wt-1 expression preceded relapse of the disease. In three of the four tested cell lines wt-1 specific transcription was demonstrated. No correlation to FAB classification, immunophenotype or response to treatment was found. Our experiments indicate wt-1 expression in the majority of AML, but not in bone marrow or PBMNC of healthy controls. Therefore, wt-1 expression may be associated with the presence of malignant blast cells and the analysis of wt-1 gene expression via PCR may be a sensitive method for the detection of leukemic blast cells.