Epstein-Barr virus (EBV) associated T/NK-cell lymphoproliferative diseases (EBV-T/NK-LPDs) are a cluster of diseases that include chronic active EBV infection (CAEBV) and aggressive NK cell leukemia (ANKL). The pathogenesis of EBV-T/NK-LPDs is largely unclear and the treatment is difficult and in most cases a hematopoietic stem cell transplantation is needed. Hemophagocytic lymphohistiocytosis (HLH) is known to affect the prognosis of patients with EBV-T/NK-LPDs. This study reports a case of a 20-year-old male patient with repeated infectious mononucleosis (IM)-like symptoms such as high fever, splenomegaly, lymphadenopathy for more than two years. The patient had a high EBV-DNA load (NK cells were the main target cells). He was first diagnosed as CAEBV. However, the disease gradually progressed and the patient developed with high ferritin, phagocytosis and monoclonal NK cells in bone marrow, pancytopenia, increased cytokines, and elevated expression of Ki-67. Also, his NK cells had abnormal immunophenotypes and impaired function. The patient had a typical clinical course of progression from CAEBV to ANKL, accompanied by HLH complications and a poor prognosis. Herein, the detailed diagnostic and differential diagnostic process of EBV infection was shown in this report.