Rosai-Dorfman disease is a rare systemic proliferative disorder of histiocytes, which mainly manifests as lymphadenopathy but also often involves extranodal tissues. Rosai-Dorfman disease was often self-limited. We hereby report a case of Rosai-Dorfman disease involving both the lymph nodes and the lungs, which was spontaneously resolved on FDG PET/CT.