A 71-year-old white man with known esophageal cancer presented for an F-18 FDG PET examination for cancer staging. There was an abnormal peripheral focus of increased activity in the upper pole of the left kidney with a maximum SUV of 11. The most likely diagnosis was felt to be either renal cell carcinoma or another hypermetabolic tumor, including metastases. A subsequent contrast-enhanced CT of the abdomen, with immediate and delayed images demonstrated a fluid/contrast level within a calyceal diverticulum accounting for the lesion on PET. This case demonstrates a potential pitfall in interpreting peripheral foci of increased uptake on PET images within the kidneys as hypermetabolic lesions. In this case, the increased uptake on the PET images is from renal excretion of radiopharmaceutical into a calyceal diverticulum rather than a hypermetabolic tumor. It is imperative that all abnormalities on FDG PET are correlated with anatomic imaging for appropriate clinical management.