The caloric requirements of the critically ill patient was defined by body composition measurements performed in thirty-five patients before and after a course of total parenteral nutrition (TPN). A positive and statistically significant (p less than 0.001) correlation was observed between the number of kcal/kg/day infused and the mean daily change in Ke, which is a measure of the body cell mass. The intercept of the regression line was 46 kcal/kg/day indicating that balance is achieved when calories are infused at this rate. Fifteen of the thirty-five patients studied received less than 46 kcal/kg/day. In this group an average of 38.1 +/- 1.5 kcal/kg/day of TPN was infused for 13.3 +/- 1.2 days. The body composition of these patients was unchanged after TPN and was characteristic of malnutrition both before and after TPN. TPN was administered at a mean rate of 57.2 +/- 2.0 kcal/kg/day for 16.7 +/- 1.8 days in the twenty patients who all received more than 46 kcal/kg/day. The body composition of these patients before TPN was also characteristic of malnutrition. However, a marked improvement of body composition was observed after the course of TPN. In all the patients in both groups the amino acids were infused at a rate that exceeded the minimal amount required to achieve nitrogen balance. The data indicate that the average patient receiving TPN requires at least 46 kcal/kg/day for balance. In the depleted patient infusion of calories in excess of this amount results in a rebuilding of the body cell mass.