1. To test the hypothesis that the presence of food in the intestine plays a role in the control of serum bilirubin levels, the effect of a 1674 kJ (400 kcal) diet, including non-absorbable material able to produce an intestinal bulk, was compared with the effect of caloric restriction alone. 2. In normal individuals ( n = 10) the rise of plasma bilirubin was comparable after the two trials. In patients with Gilbert's syndrome ( n = 18), bilirubin concentration rose to a much greater extent during the caloric restriction as compared with controls, and to a lesser extent ( P <0.001) after the bulky diet, as compared with caloric restriction alone in the same patient. 3. It is proposed that a factor originating from the intestine, perhaps a hormone, the release of which is dependent on the presence of food-bulk, plays a role in the control of hyperbilirubinaemia associated with caloric restriction.