Adult-onset Still's disease (AOSD) is a rare inflammatory disorder of unknown etiology, mainly characterized by fever, arthritis, skin rash, and raised ferritin concentration. FDG PET/CT scan of a 29-year-old woman with AOSD showed extensive lymphadenopathy, hypermetabolic splenomegaly, and increased bone marrow uptake consistent with AOSD activity. In addition, large dense lesions with high FDG uptake in the subcutaneous fat in the thighs corresponding to injection sites were seen. She had been treated with prednisone and daily subcutaneous injection of 100 mg anakinra before the scan indicating the subcutaneous lesions as injection site reactions to anakinra.