We report the case of a 75-year-old woman who underwent 18F-FDG PET/CT to evaluate the recurrence of urothelial carcinoma. PET/CT showed 18F-FDG–avid muscles and lymph nodes, mimicking recurrence. However, 18F-FDG uptake was also seen in multiple joints and subcutaneous tissue, representing an uncommon finding for recurrence. Further history taking revealed she had been pathologically diagnosed with multicentric reticulohistiocytosis (MRH) from skin biopsy, and the 18F-FDG PET/CT findings were consistent with MRH, a rare systemic inflammatory granulomatous disease of unknown etiology. Knowledge of the characteristic 18F-FDG PET/CT features of MRH is important to differentiate MRH from malignancy.