Eleven patients underwent jejunoileal bypass for morbid obesity. Serial intestinal biopsies were obtained prior to, and at timed intervals following, operation in both fasted and fat-fed states. Villus height increased asymptotically, reaching a plateau one year after operation, with an increase of 80 per cent in mean villus length. The postbypass body weight reached a plateau at 63.9 per cent of initial body weight and correlated linearly with villus height following an asymptotic curvilinear course. The time required to attain 90 per cent of total body weight loss was 15.9 months. A study of intestinal fat absorption at both the light microscopic and ultrastructural levels showed that the enlarged villi are lined along the entire villus by functionally mature epithelium capable of transporting lipid. Villus hypertrophy is an important mechanism in the plateauing of weight loss after jejunoileal bypass for morbid obesity.