Ramadan is the ninth lunar month of the Islamic calendar. During Ramadan, Muslims abstain from food and drink from dawn to sunset (fasting) to express their gratitude to God; eating and drinking is permitted only at night. Muslims typically consume two meals each day, one after sunset, and the other just before dawn. The effect of fasting during the month of Ramadan on patients with renal impairment is still a matter of controversy. This is a prospective study performed on 15 predialysis chronic kidney disease (CKD) patients and six healthy volunteers as control. They were studied during two phases: when the subjects were drinking and eating freely before the start of Ramadan, and a second phase toward the end of Ramadan. We estimated glomerular filtration rate (GFR) using DTPA dynamic renal scan, and tubular cell damage by measuring the level of N-acetyl-B-D- glucosaminidase (NAG). The change in glomerular filtration rate was -6.56 +/- 31.10 in the CKD group compared to 9.58 +/- 30.10 in the control group with no significant difference between them (p= 0.43). However, the urinary NAG percentage change was found to be significantly higher in the CKD patients compared to the control group (236 +/- 332, -49.1 +/- 60.1 respectively p= 0.03). There was a significantly positive correlation between the NAG values and the change in the blood glucose level (p=0.001), hence diabetic CKD patients should be meticulously followed during Ramadan fasting. In conclusion, fasting Ramadan may have injurious effect on the renal tubules in CKD patients. Larger studies are recommended to determine the extent of tubular injury and renal function in CKD patients during Ramadan fasting.