The phenomenon of enhanced glycolysis in tumours has been acknowledged for decades, but biochemical evidence to explain it is only just beginning to emerge. A significant hint as to the triggers and advantages of enhanced glycolysis in tumours was supplied by the recent discovery that succinate dehydrogenase (SDH) and fumarate hydratase (FH) are tumour suppressors and which associated, for the first time, mitochondrial enzymes and their dysfunction with tumorigenesis. Further steps forward showed that the substrates of SDH and FH, succinate and fumarate, respectively, can mediate a 'metabolic signalling' pathway. Succinate or fumarate, which accumulate in mitochondria owing to the inactivation of SDH or FH, leak out to the cytosol, where they inhibit a family of prolyl hydroxylase enzymes (PHDs). Depending on the PHD inhibited, two newly recognized pathways that support tumour maintenance may ensue: affected cells become resistant to certain apoptotic signals and/or activate a pseudohypoxic response that enhances glycolysis and is conveyed by hypoxia-inducible factor.