Hemophagocytic lymphohistiocytosis (HLH) is a syndrome involving an uncontrolled immune response with variable triggers. HLH is rare but highly fatal, even with proper treatment; therefore, early recognition and diagnosis are crucial for management. Although the role of F-18 fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) in HLH is poorly defined, it can provide valuable information on disease status and possible triggers. Herein, we report an F-18 FDG PET/CT study on a case of NK/T-cell lymphoma that progressed from Epstein-Barr virus-associated HLH.