Infection of resting primary human B cells by Epstein-Barr virus (EBV) results in their transformation into indefinitely proliferating lymphoblastoid cell lines (LCLs). LCL formation serves as a model for lymphomagenesis, and LCLs are phenotypically similar to EBV-positive diffuse large B-cell lymphomas (DLBCLs), which represent a common AIDS-associated malignancy. B-cell infection by EBV induces the expression of several cellular microRNAs (miRNAs), most notably miR-155, which is overexpressed in many tumors and can induce B-cell lymphomas when overexpressed in animals. Here, we demonstrate that miR-155 is the most highly expressed miRNA in LCLs and that the selective inhibition of miR-155 function specifically inhibits the growth of both LCLs and the DLBCL cell line IBL-1. Cells lacking miR-155 are inefficient in progressing through S phase and spontaneously undergo apoptosis. In contrast, three other B-cell lymphoma lines, including two EBV-positive Burkitt's lymphoma cell lines, grew normally in the absence of miR-155 function. These data identify the induction of cellular miR-155 expression by EBV as critical for the growth of both laboratory-generated LCLs and naturally occurring DLBCLs and suggest that targeted inhibition of miR-155 function could represent a novel approach to the treatment of DLBCL in vivo.