A large number of studies suggest that growth hormone (GH) modulates immune cell number and function. For example, GH administration in vitro or in animal models enhances antibody synthesis, increases T-lymphocyte proliferation, augments cytotoxic activity of natural killer cells, up-regulates priming of peritoneal and alveolar macrophages, and increases survival rates after infectious challenge. Similar effects on immune cells have been observed with insulinlike growth factor I (IGF-I), a major mediator of GH's anabolic actions. In critically ill patients, the administration of recombinant human GH improves nitrogen and mineral retention, enhances protein synthesis, and reduces urea generation. However, only limited data are available on the modulation of immune function and infection rates with GH treatment in catabolic patients. Limited studies in burn patients have shown improved wound healing and shortened hospital stay with GH therapy. Growth hormone was recently shown to improve cell-mediated immune responses and to maintain serum immunoglobulin concentrations after abdominal surgery in clinically stable patients. The immunomodulatory effects of GH administration may potentially limit septic complications in the intensive care unit setting. However, controlled studies in well-defined patient groups are needed to evaluate the potential beneficial effects of GH on immune function and infection risk in critically ill patients.