The diagnostic role of the reduced caloric intake test and phenobarbitone treatment in Gilbert's syndrome was evaluated. During fasting the increase in unconjugated serum bilirubin concentration was significantly higher in patients with Gilbert's syndrome than in normal subjects but not when compared with the increase observed in patients with acute hepatitis, which is the clinically most relevant differential diagnosis. Phenobarbital treatment significantly reduced the level of unconjugated serum bilirubin in patients with acute hepatitis or Gilbert's syndrome, but without any difference within these two groups of patients. The reduced caloric intake test and phenobarbital treatment seem to have low diagnostic specificity in Gilbert's syndrome when the differential diagnosis is that of hepatitis. The fraction of plasma unconjugated bilirubin of total bilirubin was significantly different in all three groups examined. The clinical diagnosis of Gilbert's syndrome can be established with relative certainty if the patients have a mild hyperbilirubinemia with a high fraction of unconjugated bilirubin, normal values of liver enzymes, and no overt signs of hemolysis. Liver biopsy is not mandatory.