Multicentric reticulohistiocytosis is a rare systemic inflammatory disease of unknown etiology characterized by the infiltration of histiocytes and multinucleated giant cells into multiple systems. The definitive diagnosis depends on biopsy of the affected tissues. Here, we report the F-FDG PET/CT findings of a 62-year-old man with multicentric reticulohistiocytosis. Increased FDG uptake was observed in cutaneomucosal papules, surrounding tissues of many joints, multiple muscles, and lymph nodes.