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<0.013). On the 7th, 14th, and 30th day of fasting the serum total and unconjugated bilirubin decreased to basal value. The mean total bilirubin level was 1.4±0.04 mg/dl, 1.4±0.11 mg/dl and 1.3±0.06 mg/dl on the 7th, 14th and 30th day respectively. The mean unconjugated bilirubin level was 0.9±0.04 mg/dl, 0.9±0.09 mg/dl and 0.8±0.09 mg/dl respectively. Conclusion: Compulsory fasting during Ramadan (a period of 30 days) resulted in an increase in plasma unconjugated bilirubin concentrations on the first day of fasting. Then it decreased to basal value on the 7th, 14th and 30th day in patients with Gilbert's syndrome. It is assumed that these results may be important in diagnosing Gilbert's syndrome and its clinical features in the Muslim population.