A previously healthy 3-year-old girl developed rhinorrhea, painful right neck swelling, and fever. Contrast-enhanced CT of the neck revealed cervical lymphadenopathy, for which lymphoma was considered. FDG PET/CT images showed intense activity not only in the neck but also in the thorax, abdomen, and pelvis. Biopsy was performed, and the diagnosis of Rosai-Dorfman disease was made.