Gilbert's syndrome is an inherited disorder which is characterised by unconjugated hyperbilirubinaemia. In patients with Gilbert's syndrome, both bilirubin clearance andin vitrohepatic microsomal uridine diphosphoglucuronyl transferase (UDPGT) activity are reduced. In addition, there is evidence suggesting impaired hepatic uptake of bilirubin in Gilbert's syndrome.Glucuronidation of a number of substrates appears to be impaired in Gilbert's syndrome, but the significance of the reported changes in oxidation and acetylation are less clear.