Malignant lymphocytes are characterized by their low expression of poliovirus receptor (PVR), ligand for the activating natural killer (NK) cell receptors, which may explain their insensitivity toward NK cell-based therapeutic approaches. Here, we have studied the mechanism of this defective expression of PVR. We demonstrated that the characterization of NK-insensitive cell lines was of low expression of PVR in both mRNA and surface levels, and that PVR of RAJI cells able to resist NK cells has hypermethylated promoter-associated CpG islands. After treating with 5-azacytidine (5-AZA) (ie, hypomethylation agent) and suberoylanilide hydroxamic acid (SAHA) (ie, histone deacetylase inhibitor), respectively or simultaneously, the abnormal epigenetic status became partly reversed, and the mRNA and surface expression of PVR restored. The expression restoration of the gene enhanced susceptibility of RAJI cells to NK cells but, when the RAJI cells were incubated with the specific blocking antibody for PVR's receptor, the enhanced susceptibility would diminish. Patients with acute myeloid leukemia expressed higher PVR than patients with acute lymphoblastic leukemia in both mRNA and surface levels. Epigenetic modulation of hypermethylation and histone deacetylase is involved in repressing PVR expression in malignant lymphocytes resistant to NK cells.