Background & aims: Optimal nutritional therapy for energy and protein in critically ill, mechanically ventilated patients can be defined as: providing calories matched with the measured energy expenditure and delivery of protein in an amount of 1.2-1.5 g/kg pre-admission weight/day. Several enteral nutritional products are available with different energy/protein proportions. We developed an algorithm to choose the nutritional formula that combines optimal energy and protein supply for individual patients. Methods: The energy and protein values of three nutritional formulas were used together with an aimed provision of 1.2-1.5 g protein/kg/day to construct a nomogram. From that, an algorithm followed, which was tested retrospectively in 203 mechanically ventilated patients with a normal BMI and known values for energy expenditure and weight. Results: In the nomogram cut-off points for energy/weight ratios were: 19.0-23.8 for a normal energy/high protein formula, 23.8-29.8 for a high energy/high protein formula and 30-37.5 for the normal energy/normal protein formula. The algorithm uses energy expenditure/body weight ratio of the patients to choose one of the three formulas. This resulted in an adequate provision of protein in 93% of the patients. Conclusion: The algorithm leads to provision of adequate amounts of protein and energy in the majority of critically ill, mechanically ventilated patients.