Body weight, protein, fat and water were measured before and after intravenous nutrition in 20 surgical patients. The group included both young and old patients who were suffering from a wide variety of catabolic illnesses and the period of intravenous feeding ranged from 11 to 40 days. Eight of the patients were able to take small amounts of food orally. During the period of intravenous nutrition ten patients gained significant weight although only two of these gained significant protein. Most of the weight gain was due to an increase of water. There was also a gain of water in the patients who lost weight; this water gain occurred in small daily increments over the course of treatment. It is concluded that weight gain can occur without protein gain in patients who are being fed intravenously and that body weight is not a reliable guide to changes in body protein or fat in critically ill patients receiving intravenous nutrition.