A 61-year-old-man with persistent anemia was referred for a Tc-99m-labeled red blood cell (RBC) scan to detect any source of gastrointestinal bleeding. RBC scintigraphy revealed a hypervascular mass in the left renal fossa with functional loss in that kidney, suggesting the possibility of a renal cell carcinoma (RCC). Computed tomography confirmed this. Most RCC tumors are hypervascular on conventional angiography, with a maximum incidence in men in the sixth decade. The incidental diagnosis of RCC during radionuclide imaging has been documented in previous reports, but the authors are not aware of any case of RCC diagnosed during a RBC scan. This case illustrates the importance of careful attention to the appearance of the kidneys on radionuclide scans. Noticeable asymmetric renal activity in a patient without known renal parenchymal disease warrants further investigation.