The nitrogen retention effect of branched chain enriched parenteral nutrition (BcAA) during surgical stress and polytrauma was evaluated in a prospective, randomized, double blind study in 32 patients. The doses of BcAA were 0.16, 0.3, 0.5, and 0.7 g/kg·day with a total amino acid load of 1 or 1.5 g/kg·day. It was provided in a setting of 30 glucose cal/kg·day with 7 cal/kg·day of iv fat also given to 16 patients. After the 7-day study was completed, analysis of the 4 treatment groups of 8 patients each demonstrated that there was an increased nitrogen retention effect of BcAA, that the effect was proportionate to BcAA load, that the effect started at 0.5 g of BcAA/kg·day in a setting of balanced nutritional support, and that the effect was consistent with a BcAA influence on protein synthesis. There were no complications or side-effects from high dose BcAA. Metabolic support of the stress response in ICU patients has become a clinical reality.