The metabolism of acetaminophen (paracetamol) is thought to be altered in patients with Gilbert's syndrome (GS), a chronic unconjugated hyperbilirubinemia. The underlying cause of GS is a polymorphism in the promotor region of the uridine diphosphate glucuronosyltransferase isoform 1A1 gene (UGT1A1*28), its encoded enzyme being responsible for the glucuronidation of bilirubin and presumably acetaminophen. Decreased enzyme activity results in elevated bilirubin levels and may activate various metabolic pathways leading to higher amounts of potentially hepatotoxic acetaminophen metabolites. Patients with GS might be more susceptible to unexpected side effects while taking acetaminophen and other drugs which are substrates of UGT1A1. The possibility of a correlation between glucuronidation capacity with respect to acetaminophen, UGT1A1 promotor polymorphism and the bilirubin serum level were investigated in 23 healthy male volunteers selected for UGT1A1 genotype (6 wildtypes, 9 mutants and 8 heterozygotes). One gram acetaminophen was administered p.o. and urine was collected over 2 4-hour periods. Unchanged acetaminophen and its glucuronide metabolite were determined using HPLC. The metabolic ratios unchanged acetaminophen/acetaminophen glucuronide in UGT1A1-wildtypes, heterozygotes and mutants showed no statistically significant differences. An association between metabolic ratio and serum bilirubin level could not be detected in any of the urine collection periods. These data confirm that there is no correlation between the capacity to glucuronidate acetaminophen, the UGT1A1 genotype and the bilirubin serum level. Acetaminophen is likely to be substrate of a UGT isoform other than the UGT1A1.