The ectopic expression of DNA methyltransferase 1 (DNMT 1) can transforms mammalian cells, and the inhibition of DNMT1 activity reverses that phenotypic transformation. Therefore, DNMT1 is considered to be an excellent target for therapeutic intervention. Previously, inhibition of DNMT1 was accomplished by using an antagonist or by antisense oligonucleotides. In this study, we examined the ability of the novel approach using small interfering RNA (siRNA) to disrupt the expression of DNMT1 in human non-small cell lung carcinoma A549 cells and the consequences of such an intervention. Transfection of DNMT1 siRNA decreased DNMT1 protein levels specifically and effectively. This decrease was accompanied by suppression of cell proliferation and colony-forming ability. The mechanism of this inhibition may be related to the increased levels of the cyclin dependent kinase inhibitor p21. These results suggest that the siRNA approach can be used to disrupt effectively DNMT1 activity and cancer cell growth.