An abundant amino acid in the human body, glutamine (Gln) has many important metabolic roles that may protect or promote tissue integrity and enhance the immune system. Low plasma and tissue levels of Gln in the critically ill suggest that demand may exceed endogenous supply. A relative deficiency of Gln in such patients could compromise recovery and result in prolonged illness and an increase in late mortality. This study examines this hypothesis. Using a prospective, block-randomized, double-blind treatment study design, we tested whether a Gln-containing parenteral nutrition (PN) compared with an isonitrogenous, isoenergetic control feed would influence outcome, with the endpoints of morbidity, mortality, and cost at 6 mo postintervention. In one general intensive care unit (ICU), to ensure consistency of management policies, 84 critically ill adult patients, with Acute Physiological and Chronic Health Evaluation II score > 10, requiring nutritional support received PN only if enteral nutrition was contraindicated or unsuccessful. Survival at 6 mo was significantly improved in those receiving Gln PN (24/42 versus 14/42; P = 0.049). Significantly more deaths occurred in patients requiring control PN for > 10 d (P = 0.03). The excess control deaths occurred later and those patients had had a significantly longer postintervention stay (P = 0.012) and use of ICU. In the Gln recipients, the total ICU and hospital cost per survivor was reduced by 50%. In critically ill ICU patients unable to receive enteral nutrition, a Gln-containing PN solution improves survival at 6 mo and reduces the hospital costs per survivor.