The forty-seven patients in this study suffered of massive obesity and underwent jejuno-ileal shunt operations. Seventeen of them had a second laparotomy for various reasons. At both operations, a number of measurements were made; the length of the functioning jejunal and ileal segments and, in biopsies, the intestinal villus height were determined. The second laparotomies were not performed until at least 6 months after establishment of jejuno-ileal bypass. The mean length of functioning jejunum was increased by 33% (p less than 0.05) and the mean elongation of functioning ileum by 73% (p less than 0.001). The mean villus height in functioning jejunum increased by 33% (p less than 0.001) and in functioning ileum by 70% (p less than 0.001). The mean villus height in the jejunal blind loop decreased by 30% (p less than 0.001). The demonstrated hypertrophic changes in the functioning part of the small intestine following jejuno-ileal bypass represent the morphological basis of functional intestinal adaptation. The effect of this adaptation is clearly shown in the characteristic weight diagram of an obese patient after bypass operation: the loss of weight cases, a period of stability ensues and then the patient begins to regain weight to some extent.