OBJECTIVE To investigate the effects and mechanism of Wilms' tumor (WT1) antisense oligonucleotides (AS-oligomers) on proliferation and apoptosis in myeloid leukemia cell lines. METHODS K562 and HL-60 cells were cultured in presence of WT1 oligomers. Both cell lines express WT1 gene with no p53 protein expression. Cells growth, apoptosis and expression of WT1, bcl-2 genes were analysed using 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenylmetrazolium bromide (MTT) colorimetric assay, flow cytometry and reverse transcription-polymerase chain reaction (RT-PCR) methods. RESULTS WT1 antisense oligonucleotides inhibited cellular proliferation of K562 cells and the effect was concentration-dependent. When cultured at concentration of 200 micrograms/ml oligomers, growth inhibition was 46.2% for antisense oligonucleotide cultivated group and 28.1% for sense oligonucleotide cultured group (P = 0.008) respectively. WT1 antisense oligonucleotide can induce apoptosis of K562 and HL-60 cells. Percentages of apoptotic cells in antisense oligonucleotide and sense oligonucleotide treated groups were 30.88% versus 13.62% for K562 cells and 40.15% versus 4.23% for HL-60 cells. However the growth of HL-60 cells and expression of bcl-2 gene were unaffected. CONCLUSIONS The WT1 gene is related with proliferation and apoptosis of leukemic cells. Effect of anti-apoptosis may be independent of the cellular p53 status and bcl-2 expression. WT1 gene may play an important role in leukemogenesis.