Although not mandatory for patients, many Muslims fast in Ramadan. We aimed to investigate the effects of long hours (17.5) fasting on renal functions in patients with chronic kidney disease (CKD). Stage 3-5 CKD patients with stable renal function were recruited to this prospective observational study three months ahead of Ramadan in 2015. All patients were instructed regarding possible deleterious effects of dehydration caused by fasting. Forty-five patients (mean age 66.8 10.3 years, 68.8% male) chose to fast and 49 (mean, age: 64.1 12.6 years, 51% male) chose not to fast. Clinical and laboratory data were recorded before and after Ramadan. Baseline clinical and laboratory parameters were similar in the two groups, except for higher serum creatinine and lower estimated glomerular filtration rate (eGFR) in the nonfasting group (2.22 0.99 vs. 1.64 0.41 mg/dL, P < 0.001 and 3 1.9 12.4 vs. 42.6 9.8 mL/min, P < 0.001, respectively). More than 30% elevation in serum creatinine after Ramadan occurred in 8.8% and 8.1% of fasting and nonfasting patients, respectively (P = 0.9). More than 25% drop eGFR after Ramadan was noted in seven (15.5%) and six (12.2%) fasting and nonfasting patients, respectively (P = 0.642). Patients with >= 25% drop in eGFR (13 vs. 81) were older (72.3 8.3 years vs. 64.3 11.7 years, P = 0.020) and more frequently using diuretics (69.2% vs. 35.8%, P = 0.023). In multiple linear regression analysis, only advanced age was found to be associated with >= 25% drop in eGFR after Ramadan in the fasting group. Fasting during Ramadan was not associated with increased risk of declining in renal functions in patients with Stage 3-5 CKD. However, elderly patients may still be under a higher risk.