It has been known for 30 years that Epstein-Barr virus (EBV), a ubiquitous human herpesvirus, is the etiologic agent of acute infectious mononucleosis and is closely associated with the genesis of Burkitt's lymphoma and undifferentiated nasopharyngeal carcinoma. Recent studies have demonstrated that EBV is also implicated in a variety of other diseases, such as EBV-associated hemophagocytic syndrome, chronic active EBV infection, T-cell lymphoma, natural killer cell leukemia/lymphoma, lymphoproliferative diseases in immunocompromised hosts, Hodgkin's disease, pyothorax-associated B-cell lymphoma, smooth-muscle tumors, and gastric carcinoma. Thus, the virus continues to attract worldwide attention, and it is now appropriate for a reappraisal of the relation between EBV and human diseases. This review summarizes the recent progress in research on EBV and the clinical findings of EBV-associated diseases and provides a basis for the development of new therapeutic strategies.