Twenty severely brain-injured patients with Glasgow Coma Scale scores of 4-9 were prospectively randomized to receive one of two standard amino acid formulas, starting with the first day of hospital admission up to day 14 postinjury. Formula 2 (patient group 2) had 54 percent more leucine, 53 percent more isoleucine, 74 percent more valine, 28 percent less phenylalanine, 31 percent less methionine, 111 percent more proline, 38 percent less alanine, and 38 percent less glycine than formula 1 (patient group 1). Groups 1 and 2 received statistically equal overall mean parenteral nutrition calories and protein (2173 +/- 147 vs. 2059 +/- 143 kcal, and 77 +/- 12 vs. 83.1 +/- 6 g, respectively). There was a significant difference in overall mean urinary urea nitrogen excretion (group 1 = 24.6 +/- 1.3 vs. group 2 = 18.3 +/- 1.1, p = 0.02) and nitrogen balance (group 1 = -8.0 +/- 2.1 vs. group 2 = +1.8 +/- 1.2, p = 0.01). Mean overall isoleucine values were significantly higher in group 2 (overall mean 77 mumol/L vs. 62 mumol/L, p = 0.04). Phenylalanine levels were significantly higher in group 1 (107 mumol/L) versus group 2 (82 mumol/L) patients (p = 0.01). Arginine levels were significantly higher in group 1 (78 mumol/L) versus group 2 (49 mumol/L) patients (p = 0.0002). This observation suggests that some standard intravenous amino acid formulas may be more apt to promote positive nitrogen balance than others.