Background/Aims: The fractional excretion of sodium (FeNa) may be helpful in establishing the cause of acute renal failure. This study was performed to determine the influence of the glomerular filtration rate (GFR), sodium intake, and tubular function on FeNa in children without renal failure. Methods: In this single institute cross-sectional study, 24-h-urine collections from patients (4-18 years of age, GFR >60 mL/min/1.73 m2) were used when considered reliable, and analyzed to determine sodium excretion, creatinine clearance and FeNa. The influence of tubular function was studied in 5 patients with generalized tubular dysfunction. Results: Based on data from 761 patients, a multiple regression formula was designed based on GFR and sodium excretion that predicted over 80% of the variation in FeNa (R2 = 0.824, p < 0.001). Using this formula, the predicted FeNa was significantly lower than the measured FeNa in the children with tubular dysfunction. Conclusion: FeNa depends on GFR and sodium intake, and tubular function cannot be assessed without possessing adequate knowledge on all factors. Therefore, no normal range or cut-off value for FeNa to assess tubular function can be given, and doing so is a misapplication of this equation.