Hyperpolarized [1-13C]pyruvate MRS was conducted to directly monitor the transition of energy metabolism in tiny multicellular tumor spheroids as a model of early tumorigenesis. As compared with normal tumor cells, the lactate formation from pyruvate was significantly amplified in the tumor spheroids, even in a few hundred micrometer i.d. of smaller ones with no blood vessel formation. These results imply that formation of the sterical structure itself causes the transition of energy metabolism from mitochondria to cytoplasm in tumor tissues. HP [1-13C]pyruvate MRS may thus allow detection of early tumorigenesis by targeting the increased aerobic glycolysis in the initial stage, if much higher performance of HP measurement is realized.