We describe a case of fever of unknown origin (FUO), renal failure, and pancytopenia.Initially, lymph proliferative disorder was suspected; therefore, bone marrow biopsy and 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography/computed tomography (PET/CT) were performed. Bronchoscopy and lung biopsy were performed because of abnormal FDG uptake in both lung fields. Taken together with the laboratory and histological results, sarcoidosis with bone marrow invasion was confirmed. The patient was discharged following favorable response to corticosteroid therapy.Sarcoidosis may present as FUO without typical specific presentations in the skin or lung.Combined 18F-FDP PET/CT helped identify the biopsy site and confirmed sarcoidosis diagnosis.