Epstein–Barr virus (EBV), implicated in numerous human diseases, including lymphoid malignancies, persistently infects peripheral B cells and transforms them into lymphoblastoid cell lines. Here we found that EBV equally infected B cells from patients with X-linked hyper IgM syndrome and those from healthy donors; however, it hardly transformed X-linked hyper IgM syndrome B cells, because of the dysfunctional gene of CD40 ligand (CD40L) of the patients. Unlike CD40, CD40L is not usually expressed on B cells. However, we found that EBV infection of normal B cells induced CD40L expression as a critical effector in host cell transformation and survival. Moreover, chronic active EBV infection of peripheral T cells, implicated in T cell malignancies, was associated with ectopic expression of CD40, and, in Jurkat T cells, EBV infection induced CD40 expression. These results suggest that EBV infection induces CD40L/CD40 signaling in host cells, which appears to play an essential role in its persistent infection and malignancies of lymphocytes.