Gilbert's syndrome (GS) is characterised by the existence of chronic mild unconjugated hyperbilirubinaemia. The value of rifampicin as a provocative test for the diagnosis of GS was evaluated and compared with a fasting test. Twenty-two patients with GS, 15 patients with chronic liver disease and 20 healthy controls were included. Both rifampicin and fasting tests were applied to all subjects. In the fasting test, the subjects were given a 400 calorie/day diet for 24 hours; in the rifampicin test, unconjugated bilirubin levels was measured before and four hours after taking 600 mg of rifampicin. Both tests achieved a significant increase in mean unconjugated bilirubin levels in patients with GS but not in the controls. The sensitivity and specificity of a rifampicin test in the diagnosis of GS were comparable with the fasting test. However, both tests caused a significant increase in unconjugated bilirubin levels in nearly half the patients with chronic liver disease. A rifampicin test may be used in the diagnosis of suspected GS instead of a fasting test, as it is simpler and more practical. However, its specificity for GS is not sufficient, because it also causes an increase in unconjugated bilirubin levels in some patients with chronic liver disease.