The effect of Ramadan fasting on fasting serum glucose (FPG) is still a matter of controversy. One hundred and fifteen healthy volunteers fasted for > or = 25 days during Ramadan. Blood samples were taken 1 week before Ramadan and on the 14th and 28th day of Ramadan. Serum glucose was determined by the glucose oxidase method. FPG decreased from 88.4 + 9.0 mg/dl to 62.9 +/- 7.7 mg/dl (p < 0.001). Men and women both experienced a significant drop in FPG. Calorie intake decreased in all groups (p < 0.001) and was positively correlated with decreasing FPG. FPG decreases with Ramadan fasting and has no serious adverse effect on fasting adults.