Involvement of Epstein-Barr virus (EBV) has long been known in the development of various tumor-forming proliferating diseases, such as nasopharyngeal carcinoma in adults. However, in children and young adults more attention should be focused on systemic, severe type EBV-related diseases, such as fatal infectious mononucleosis, hemophagocytic syndrome, or chronic active EBV infection (CAEBV). These disorders show the typical clinical features of hemophagocytic lymphohistiocytosis (HLH). Although viral infectious diseases are mostly taken care of by infectious disease specialists, pediatric hemato-oncologists need to intervene in the treatment of this kind of disease because of their clonal and neoplastic disease characteristics and of their hematologically problematic, rapid, and life-threatening clinical courses.