Background: Critically ill patients typically receive 60 of estimated calorie requirements. Objectives: We aimed to determine whether the substitution of a 1.5-kcal/mL enteral nutrition solution for a 1.0-kcal/mL solution resulted in greater calorie delivery to critically ill patients and establish the feasibility of conducting a multicenter, double-blind, randomized trial to evaluate the effect of an increased calorie delivery on clinical outcomes. Design: A prospective, randomized, double-blind, parallel-group, multicenter study was conducted in 5 Australian intensive care units. One hundred twelve mechanically ventilated patients expected to receive enteral nutrition for >2 d were randomly assigned to receive 1.5 (n = 57) or 1.0 (n = 55) kcal/mL enteral nutrition solution at a rate of 1 mL/kg ideal body weight per hour for 10 d. Protein and fiber contents in the 2 solutions were equivalent. Results: The 2 groups had similar baseline characteristics (1.5 compared with 1.0 kcal/mL). The mean (?SD) age was 56.4 ? 16.8 compared with 56.5 ? 16.1 y, 74 compared with 75 were men, and the Acute Physiology and Chronic Health Evaluation II score was 23 ? 9.1 compared with 22 ? 8.9. The groups received similar volumes of enteral nutrition solution [1221 mL/d (95 CI: 1120, 1322 mL/d) compared with 1259 mL/d (95 CI: 1143, 1374 mL/d); P = 0.628], which led to a 46 increase in daily calories in the group given the 1.5-kcal/mL solution [1832 kcal/d (95 CI: 1681, 1984 kcal/d) compared with 1259 kcal/d (95 CI: 1143, 1374 kcal/d); P <0.001]. The 1.5-kcal/mL solution was not associated with larger gastric residual volumes or diarrhea. In this feasibility study, there was a trend to a reduced 90-d mortality in patients given 1.5 kcal/mL [11 patients (20 ) compared with 20 patients (37 ); P = 0.057]. Conclusions: The substitution of a 1.0-with a 1.5-kcal/mL enteral nutrition solution administered at the same rate resulted in a 46 greater calorie delivery without adverse effects. The results support the conduct of a large-scale trial to evaluate the effect of increased calorie delivery on clinically important outcomes in the critically ill.