Objective: We investigated the effect of compulsory fasting on unconjugated hyperbilirubinemia in ten adult patients with Gilbert's syndrome during Ramadan (a fasting period of 30 days). Methods: At 4 a.m. and 8 p.m. they were given meals which contained a total of approximately 2500 cal. After the 4 a.m. baseline measurement was taken, the patients did not eat or drink anything for 16 hr for religious reasons. The serum total and indirect bilirubin concentrations were measured every week during this 4 week period of compulsory fasting. Results: Prefasting basal mean serum bilirubin concentrations were: total bilirubin 1.4±0.06 mg/dl, conjugated bilirubin 0.4±0.03 mg/dl, unconjugated bilirubin 1.0±0.05 mg/dl. On the first day after a 16 h fasting the values were: total bilirubin 2.6±0.06 mg/dl, conjugated bilirubin 0.4±0.03 mg/dl and unconjugated bilirubin 2.1±0.12 mg/dl. In this period, conjugated bilirubin remained unchanged while total and unconjugated bilirubin significantly increased compared to the basal value (p