Although sunitinib shows a high response rate in patients with untreated metastatic renal cell carcinoma (mRCC), quite a few patients show no therapeutic effect. Therefore, it is crucial to distinguish the patients who respond to sunitinib from those who do not as early as possible after the administration of the therapy. We herein report a case of mRCC in which 11C-acetate (AC) positron emission tomography (PET) showed an early therapeutic effect of sunitinib treatment 4 weeks after its administration.