Side population (SP) cells within tumors are a small fraction of cancer cells with stem-like properties that can be identified by flow cytometry analysis based on their high ability to export certain compounds such as Hoechst 33342 and chemotherapeutic agents. The existence of stem-like SP cells in tumors is considered as a key factor contributing to drug resistance, and presents a major challenge in cancer treatment. Although it has been recognized for some time that tumor tissue niches may significantly affect cancer stem cells (CSCs), the role of key nutrients such as glucose in the microenvironment in affecting stem-like cancer cells and their metabolism largely remains elusive. Here we report that SP cells isolated from human cancer cells exhibit higher glycolytic activity compared to non-SP cells. Glucose in the culture environment exerts a profound effect on SP cells as evidenced by its ability to induce a significant increase in the percentage of SP cells in the overall cancer cell population, and glucose starvation causes a rapid depletion of SP cells. Mechanistically, glucose upregulates the SP fraction through ATP-mediated suppression of AMPK and activation of the Akt pathway, leading to elevated expression of the ATP-dependent efflux pump ABCG2. Importantly, inhibition of glycolysis by 3-BrOP significantly reduces SP cells in vitro and impairs their ability to form tumors in vivo. Our data suggest that glucose is an essential regulator of SP cells mediated by the Akt pathway, and targeting glycolysis may eliminate the drug-resistant SP cells with potentially significant benefits in cancer treatment. Accumulating evidence suggests that tumors of various tissue origins, including the brain, breast, and lung, contain a small subpopulation of special cells with stem-like properties, often referred to as cancer stem cells (CSCs) or tumor-initiating cells. [1][2][3][4][5][6][7] In addition to the ability of CSCs to self-renew and initiate tumor formation, one important biochemical feature of CSCs is their ability to export certain toxic compounds and resistance to many chemotherapeutic agents due in part to their high expression of ATP-dependent efflux pump ABCG2, their increased DNA repair capacity, and activation of survival pathways. [8][9][10] The increase in expression of ABCG2 also confers CSCs the ability to effectively export the DNA-binding dye Hoechst 33342 out of the cells, leading to a low retention of the fluorescent signal in these cells, which appear at the low-left corner in flow cytometry analysis and thus are known as 'side population' or SP cells. 2,11 As SP cells can be readily identified by flow cytometry in a quantitative manner, the measurement of SP cells has been widely used as a quantitative assay for the relative number (%) of stem-like cancer cells in the bulk of the overall cancer cell population. Importantly, as this assay is functionally based on the ability of SP cells to export Hoechst 33342 and certain toxic compounds, it is also a quantitative analysis of the subpopulatio...