It is well documented that disturbances in mitochondrial function are associated with rare childhood disorders and possibly with many common diseases of ageing, such as Parkinson's disease and dementia. There has also been increasing evidence linking mitochondrial dysfunction with tumorigenesis. Recently, heterozygous germline mutations in two enzymes of the Krebs tricarboxylic acid cycle (TCA cycle) have been shown to predispose individuals to tumours. The two enzymes, fumarate hydratase (FH) and succinate dehydrogenase (SDH), are ubiquitously expressed, playing a vital role in adenosine triphosphate (ATP) production through the mitochondrial respiratory chain. Germline mutations in FH are associated with leiomyomatosis and renal cell carcinoma, whilst SDH mutations are associated with predisposition to paraganglioma (PGL) and phaeochromocytoma (PCC). At present, there are few data to explain the pathway(s) involved in this predisposition to neoplasia through TCA cycle defects. We shall review the mechanisms by which mutations in FH and SDH might play a role in tumorigenesis. These include pseudo-hypoxia, mitochondrial dysfunction and impaired apoptosis, oxidative stress and anabolic drive. All of these mechanisms are currently poorly defined. To date, FH and SDH mutations have not been reported in non-familial leiomyomata, renal cancers, PCCs or PGLs. It remains entirely possible, however, that the underlying mechanisms of tumorigenesis in these sporadic tumours are the same as those in the Mendelian syndromes.