The influence of dietary composition on the unconjugated hyperbilirubinaemia of Gilbert's syndrome was studied in 29 patients. After a period on a normal diet (10 MJ) an intravenous infusion of 40% glucose (8-4 MJ) together with a 1-6 MJ oral diet for two days resulted in an increment in plasma bilirubin concentration of 127 +/- 18% (mean +/- SEM) above the basal level. Both the administration of intravenous Intralipid 20% and the return to a normal diet caused a prompt reversal of this glucose effect. An increment of 135 +/- 10% in plasma bilirubin concentration was obtained when a standard "fasting" diet (1-6 MJ) was given for two days. When the lipid content of this "fasting" diet was increased from 33% to 85%, the rise in plasma bilirubin was only 49 +/- 19%. A 10 MJ oral diet for three days, which contained most of its energy content as carbohydrate and only 0-6% as lipid, produced a 76 +/- 12% increase in plasma bilirubin concentration. When the lipid content of the diet was increased to 9% of the energy intake no significant change from the basal level was observed. These findings support the hypothesis that the hyperbilirubinaemia associated with both carbohydrate feeding and fasting is attributable, at least in part, to lipid withdrawal. Although a restricted dietary intake or the parenteral administration of lipid-free solutions has a marked effect on the hyperbilirubinaemia of patients with Gilbert's syndrome, normal daily variation in dietary composition is unlikely to cause a significant change. The influence of different feeding regimes on neonatal hyperbilirubinaemia requires investigation.