Energy requirements of pediatric intensive-care patients are unknown due to the difficulty of measuring total energy expenditure in free-living conditions. We investigated energy expenditure and body composition in stable pediatric intensive-care patients receiving long-term ventilatory support. Total energy expenditure and total body water were measured in 10 such patients using the doubly-labeled water method. The patients had significantly lower energy expenditure than healthy children of the same age. Relative to length, fat-free mass deposition was significantly lower, and fat deposition was significantly greater, than in healthy subjects. In general, total energy requirements of these patients are significantly reduced as compared to healthy children, which can be attributed to their lower activity levels and their reduced muscle mass. Although they gain weight similar to healthy children, their weight gain is disproportionately higher in fat. However, one patient with myofibromatosis contradicted this general pattern.