Tehran University of Medical Sciences Case Reports in Clinical Practice 2538-2683 8 2 2023 11 18 64 67 Maassoumeh Akhlaghi Rheumatology Research Center, Tehran University of Medical Sciences, Tehran, Iran. Kiarash Kazemi Rheumatology Research Center, Tehran University of Medical Sciences, Tehran, Iran. Soheila Sobhani Rheumatology Research Center, Tehran University of Medical Sciences, Tehran, Iran. 2022 11 20 2023 01 31 Infectious mononucleosis (IM), usually caused by Epstein-Barr virus (EBV), is not rare among populations. Fever, pharyngotonsilitis and lymphadenopathy are its major presentations. Since EBV mainly targets lymphomononuclear cells and the reticuloendothelial system, it might be mistaken with hematological malignancies. The case presented in this article showed a very high similarity to an acute lymphoblastic leukemia, which complicated the diagnostic work-up. The histopathological data and clinical course of IM patients are not usually distinctive; hence, early work-up for EBV-associated IM is suggested. https://crcp.tums.ac.ir/index.php/crcp/article/view/758