Workup of fever of unknown (FUO) is challenging due to non-specific clinical manifestations. We report the case of a patient who presented with FUO for two months. Further workup with laboratory test, image study and tissue sample confirmed the diagnosis of lymphoma. Our case illustrates the importance of careful interpretation of laboratory test during the workup of FUO. For example, abnormal ferritin and hemoglobin levels are associated with higher risk of hematology maligancy.  (J Intern Med Taiwan 2022; 33: 383-388)