BACKGROUND: Arginine is considered an essential amino acid during critical illness in children, and supplementation of arginine has been proposed to improve arginine availability to facilitate nitric oxide (NO) synthesis. Protein-energy-enriched enteral formulas (PE-formulas) can improve nutrient intake and promote anabolism in critically ill infants. However, the effect of increased protein and energy intake on arginine metabolism is not known. OBJECTIVE: We investigated the effect of a PE-formula compared with that of a standard infant formula (S-formula) on arginine kinetics in critically ill infants. DESIGN: A 2-h stable-isotope tracer protocol was conducted in 2 groups of critically ill infants with respiratory failure because of viral bronchiolitis, who received either a PE-formula (n = 8) or S-formula (n = 10) in a randomized, blinded, controlled setting. Data were reported as means +/- SDs. RESULTS: The intake of a PE-formula in critically ill infants (aged 0.23 +/- 0.14 y) resulted in an increased arginine appearance (PE-formula: 248 +/- 114 mumol kg-1 h-1; S-formula: 130 +/- 53 mumol kg-1 h-1; P = 0.012) and NO synthesis (PE-formula: 1.92 +/- 0.99 mumol kg-1 h-1; S-formula: 0.84 +/- 0.36 mumol kg-1 h-1; P = 0.003), whereas citrulline production and plasma arginine concentrations were unaffected. CONCLUSION: In critically ill infants with respiratory failure because of viral bronchiolitis, the intake of a PE-formula increases arginine availability by increasing arginine appearance, which leads to increased NO synthesis, independent of plasma arginine concentrations. This trial was registered at www.trialregister.nl as NTR515.