A prominent feature of immortal cells and stem cells is their resistance to oxidative stress. By contrast, primary cells undergo senescence when grown for extended periods in tissue culture or exposed to agents that increase production of reactive oxygen species (ROS). We recently found that enhanced glycolysis enables primary mouse cells to avoid senescence, by protecting them from oxidative damage, and that immortal ES cells have intrinsically high levels of glycolysis. Thus, in cancer cells, immortality could be the driving force behind the Warburg effect. © 2005 Elsevier Ltd. All rights reserved.