Glutamine has many important metabolic roles that may protect or promote tissue integrity and enhance the immune system. The normal abundance of glutamine has meant that it has not been considered necessary to include glutamine in traditional parenteral feeds. However, low plasma and tissue levels of glutamine (Gln) in the critically ill suggest that demand may exceed endogenous supply. A relative deficiency of glutamine in such patients could compromise recovery, result in prolonged illness, and an increase in late mortality. The few percent of the most critically ill intensive care patients who are unable to tolerate enteral nutrition are especially at risk since they have increased demands for glutamine yet lack an exogenous supply. Such patients undergo considerable skeletal muscle wasting compromising glutamine supply further. In a prospective, randomised double blind clinical study of 84 patients with a high mortality due to multiple organ failure requiring parenteral feeding a significant improvement in six-month survival was observed in the group supplemented with glutamine 24/42 versus isonitrogenous, isoenergetic control 14/42, P = 0.049.