The nonessential amino acid glutamine has recently been the focus of extensive scientific interest because of its importance in cell and tissue cultures and its physiologic role in animals and humans. Glutamine appears to be a unique amino acid, serving as a preferred respiratory fuel for rapidly proliferating cells, such as enterocytes and lymphocytes; a regulator of acid-base balance through the production of urinary ammonia; a carrier of nitrogen between tissues; and an important precursor of nucleic acids, nucleotides, amino sugars, and proteins. Abundant evidence suggests that glutamine may become a "conditionally essential" amino acid in the critically ill. During stress the body's requirements for glutamine appear to exceed the individual's ability to produce sufficient amounts of this amino acid. Provision of supplemental glutamine in specialized enteral or parenteral feeding may enhance nutritional management and augment recovery of the seriously ill while minimizing hospital stay.