Clear cell renal cell carcinoma (ccRCC) is the most common form of human kidney cancer. Histological and molecular analyses suggest that ccRCCs have significantly altered metabolism. Recent human studies of lung cancer and intracranial malignancies demonstrated an unexpected preservation of carbohydrate oxidation in the tricarboxylic acid (TCA) cycle. To test the capacity of ccRCC to oxidize substrates in the TCA cycle, we infused ¹³C-labeled fuels in ccRCC patients and compared labeling patterns in tumors and adjacent kidney. After infusion with [U-¹³C]glucose, ccRCCs displayed enhanced glycolytic intermediate labeling, suppressed pyruvate dehydrogenase flow, and reduced TCA cycle labeling, consistent with the Warburg effect. Comparing ¹³C labeling among ccRCC, brain, and lung tumors revealed striking differences. Primary ccRCC tumors demonstrated the highest enrichment in glycolytic intermediates and lowest enrichment in TCA cycle intermediates. Among human tumors analyzed by intraoperative ¹³C infusions, ccRCC is the first to demonstrate a convincing shift toward glycolytic metabolism.