The metabolic responses to branched chain amino acid enriched total parenteral nutrition were studied in surgical intensive care patients with documented severe catabolism. Twenty-four patients were randomised to receive total parenteral nutrition with either 50% (BCAA) or 15% (CONTROL) of its amino acid content as branched chain amino acids. The daily intake of nitrogen was 0.24 g per kg b. wt and that of energy, at steady state, 45 kcal per kg b. wt. Total parenteral nutrition resulted in similar nitrogen balances (Day 1 BCAA: -0.07 +/- 0.09, CONTROL: -0.05 +/- 0.10; Day 4 BCAA: -0.12 +/- 0.07, CONTROL: -0.06 +/- 0.09; Day 5 BCAA: -0.07 +/- 0.08, CONTROL: -0.05 +/- 0.08 g/kg b. wt day; mean +/- S.D.) in both treatment groups throughout the study. The urinary excretion of 3-methylhistidine was higher before feeding in the control group but the rates were similar during total parenteral nutrition in both groups. There were no differences between the groups in the decrease of serum prealbumin during the study. We conclude that the proposed benefits, if any, from enriching nutritional regimens with branched chain amino acids are unlikely to be of major therapeutic relevance in severe catabolism.