A group of eleven subjects with Gilbert's syndrome was characterized by conventional tests and determination of bilirubin and its conjugates in plasma by alkaline methanolysis and thin layer chromatography. After a 1 g dose of paracetamol h.s. the drug and its metabolites were measured by high performance liquid chromatography (HPLC) in the overnight 8-h urine sample. The amounts of paracetamol and of its metabolites recovered in urine were almost identical with those found in the control group (n = 10). The glucuronide:paracetamol ratio, which is considered to be an index of glucuronidation, was not correlated with the fraction of bilirubin present in plasma as glucuronides. These data do not suggest that in subjects with Gilbert's syndrome therapeutic doses of paracetamol are associated with an increased risk for hepatic or systemic toxicity.