1. After jejunoileostomy patients rapidly loose body weight. This loss of weight may turn out to be too small in cases where there is a reflux. 2. There is a direct relationship between the rapidity of the loss of body weight and the duration of gastro-intestinal passage. The shorter the duration of passage, the quicker the patient's loss of body weight. 3. Duration of passage remains approximately constant during the first 9 months after jejunoileostomy as shown by the evaluation of 52 patients. From the 15th month onward the duration of passage rises significantly and reaches 2-3 times its initial value from the 18th month onward. Simultaneously, a cessation of the loss of body weight is to be observed owing to an adaptation to the malabsorption.