Abstract: Rosai-Dorfman disease (RDD), also known as sinus histiocytosis with massive lymphadenopathy, is a rare histiocytic disease. Typically, a patient with RDD presents with painless bilateral cervical lymphadenopathy. The involvement of extranodal organs, such as the skin, upper respiratory tract, and bone, may also occur. Bone involvement occurs in approximately 8% of patients with RDD. Obtaining an affirmatory diagnosis by non-specific radiological findings is difficult. The final diagnosis can be achieved by biopsy and pathological examination of the lesion. The ¹⁸F-FDG PET/CT results of a young patient aged 36 with RDD who presented with intermittent pain in the right leg was reported in this article. This case illustrates the usefulness of functional fused modality in differentiating diagnosis and in identifying the extent of this disease and the appropriate sites for biopsy.