BACKGROUND & AIMS: Nutritional support improves outcome in critically ill infants but is impeded by fluid restriction, gastric intolerance and feeding interruptions. Protein and energy-enriched infant formulas may help to achieve nutritional targets earlier during admission and promote anabolism. METHODS: Randomized controlled design. Infants with respiratory failure due to RSV-bronchiolitis received a protein and energy-enriched formula (PE-formula, n=8) or a standard formula (S-formula, n=10) during 5 days after admission. Primary outcome: nutrient delivery, energy and nitrogen balance and plasma amino acid concentrations. Secondary outcome: tolerance and safety. RESULTS: Nutrient intakes were higher in PE fed infants and met population reference intake (PRI) on day 3-5 whilst in S-fed infants PRI was met on day 5 only. Cumulative nitrogen balance (cNB) and energy balance (cEB) were higher in PE-infants compared to S-infants (cNB: 866+/-113 vs. 296+/-71mg/kg; cEB: 151+/-31 and 26+/-17kcal/kg, both P<0.01). Essential amino acid levels were higher in PE-infants but within reference limits whereas below these limits in S-infants. Both formulas were well tolerated. CONCLUSIONS: Early administration of a protein and energy-enriched formula in critically ill infants is well tolerated, promotes a more adequate nutrient intake and improves energy and nitrogen balance without adverse effects.