A 70-year-old man was incidentally found to have an occupying lesion in the left renal pelvis by ultrasonography during a routine health examination. CT images suggested renal cell carcinoma. FDG PET/CT scan was acquired for staging. The images did not reveal any hypermetabolic metastases. However, the metabolic activity of the lesion in the right renal pelvis was not unambiguously determined because of interference of radioactive urine. C-acetate PET/CT, on the other hand, clearly revealed increased activity in the left renal pelvis. Pathological examination demonstrated solitary extraosseous plasmacytoma.