Changes in body weight (absolute and percent), overweight index, and body surface area were examined in 27 grossly obese subjects after jejunal-ileal by-pass. Curves were steeper and linear for 3-4 months and then flattened out for the remainder of the twelve-month period of observation. They were parallel to each other even when a division was made between patients weighing more and less than twice their ideal weight. This shows that if a person weighs more than 120 Kg his weight loss curve will depend on the time that has elapsed since surgery. It is virtually independent of the start value.