Although total parenteral nutrition usually includes lipids, traditional long-chain triglyceride (LCT) emulsions do not fulfil the energy-providing role allotted to them. The special properties of medium-chain triglycerides (MCTs) and fatty acids led to replacement of part of the infused LCTs by MCTs. The present review shows that: 1. MCT/LCT emulsions are as safe and as well tolerated as the traditional emulsions, and contain enough essential fatty acids to meet patients' needs. 2. Relative to LCT emulsions, MCT/LCT emulsions exhibit a number of differences: * More rapid clearance from the circulation. Lipoprotein lipase and hepatic lipase hydrolyse them preferentially. * Decreased liability to be deposited as fat, in adipose tissue and liver. They do not overload the reticula-endothelial system, which may better preserve its capacity to phagocytose bacteria. * More rapid and complete oxidation, Faster energy provision for all tissues, even though a small part is dissipated in a clinical non-relevant thermogenesis and by o-oxidation. They are ketogenic if infused alone. * Concomitant administration of glucose does not influence their clearance rate, only slightly decreases their oxidation rate, but prevents the acceleration of ketogenesis. Two other properties of MCT/LCT emulsions are probable, though not confirmed: * exchanges of lipids between artificial fat particles and plasma lipoproteins may be less with these emulsions than with LCTs, though it is not yet known what effect diminished disturbance of lipoprotein homeostasis has on the organism. * The nitrogen-sparing effect of a TPN regimen containing MCTs/LCTs seems better than a regimen providing LCTs only.