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Diffuse large B-cell lymphoma (DLBCL) is an aggressive malignancy and the most common subtype of non-Hodgkin lymphoma in China. However, many cases still remain biologically and clinically heterogeneous, indicating that the DLBCL mechanism remains unclear. MicroRNAs (miRNAs) are critically responsible for lymphomagenesis. We found that plasma miR-21 level was significantly higher in B-cell lymphoma. However, the exact contribution of miR-21 in DLBCL remains unknown.To determine the function and mechanism of miR-21 in DLBCL, miR-21 and phosphatase and tensin homolog (PTEN) expressions were examined through real-time PCR and immunohistochemical methods. Moreover, the effects of antisense oligonucleotide (ASO) targeting miR-21 (ASO-21) were observed in DLCBL cell line.MiR-21 expressions in cell line and tissues of patients were significantly higher than those in normal controls, which were inversely correlated with PTEN expression. MiR-21 expression was significantly higher in stage III/IV patients than in stage I/II patients. PTEN protein was expressed positively in only 6 patients with DLBCL (6/26). MiR-21 expression level in the PTEN-negative group was 11.73 (2.13–64.29), which was significantly higher than that in the PTEN-positive group (1.04, 0.67–15.15; P = .038). After down-regulating the miR-21 expression, apoptosis of DLBCL cells increased and PTEN protein was up-regulated in ASO-21-treated cells compared with SCO-21-treated cells by western blot.These results suggested that miR-21 affects apoptosis of lymphoma cells by regulating the expression of PTEN in DLBCL, which may be associated with increased poor prognosis for DLBCL patients and represents a useful approach for DLBCL treatment.