Protein loss leading to reduced lean body mass is recognised to contribute to the high levels of morbidity and mortality seen in critical illness. This prospective, randomised, controlled study compared the effects of conventional parenteral nutrition (TPN), glutamine supplemented (0.4 g kg^-1 day^-1) TPN (TPNGLN) and TPNGLN with combined growth hormone (GH, 0.2 IU kg^-1 day^-1) and insulin-like growth factor-I (IGF-I, 160 µg kg^-1 day^-1) on protein metabolism in critical illness. Nineteen mechanically ventilated subjects (64±3, years, BMI 23.8±1.3, kg/m², mean±SE) were initially studied in the fasting state (study 1) and subsequently following three days nutritional ± hormonal support (study 2). All had recently been admitted to the ICU and the majority were post-emergency abdominal surgery (APACHE II 17.5±1.0). Protein metabolism was assessed using a primed constant infusion of [1-¹³C] leucine. Conventional TPN contained mixed amino acids, Intralipid and 50% Dextrose. TPNGLN unlike TPN alone resulted in an increase in plasma glutamine concentration (approximately 50%, p<0.05). Both TPN and TPNGLN decreased the rate of protein breakdown (TPN; 15 %, p<0.002; TPNGLN 16%, p<0.05) but during these treatments the patients remained in a net negative protein balance. Combined treatment with TPNGLN + GH/IGF-I increased plasma IGF-I levels (10.3±0.8 vs 48.1±9.1, nmol/l, study 1 vs study 2, p<0.05), and in contrast to therapy with nutrition alone, resulted in net protein gain (-0.75±0.14 vs 0.33±0.12, grams protein kg^-1 day^-1, study 1 vs study 2, p<0.05). Therapy with both GH and IGF-I in addition to glutamine supplemented TPN, unlike nutrition alone, resulted in net positive protein balance in a group of critically ill patients.