FDG PET/CT was performed in a 30-year-old woman to detect the source of fever of unknown origin. The images showed widespread abnormal activity, consistent with lymphoma. However, lymph node biopsy revealed only inflammation, and a diagnosis of familial hemophagocytic lymphohistiocytosis was eventually made after familial hemophagocytic lymphohistiocytosis-specific gene was detected. After proper therapy, a follow-up FDG PET/CT examination 2 months later showed resolution of most of the previously seen lesions.