Proliferation of the lymphoid system should arouse suspicion of a potentially serious illness. We present a 4.5-year-old boy who developed fever, vomiting, diarrhea, lymphadenopathy, hepatosplenomegaly, lymphocytosis, anemia, thrombocytopenia, and increased liver enzymes. Lymph node and bone marrow biopsies showed lymphoproliferation, Epstein-Barr virus (EBV) infection, and hemophagocytosis leading to the diagnosis of hemophagocytic lymphohistiocytosis (HLH). Chemotherapy was initiated for HLH with dexamethasone, etoposide, and cyclosporine. Because of a high level of EBV viremia, rituximab was added a few days later and resulted in a remarkable drop in the EBV in the circulation but not in the cerebrospinal fluid. However, the patient succumbed to encephalitis, pneumonia, and cardiopulmonary failure. Autopsy revealed the presence of EBV in the brain, indicating the ineffectiveness of rituximab therapy in treating central nervous system infection with EBV.