Fractional excretion of sodium (FENa) has been said to be the most sensitive index for differentiating prerenal failure (PRF) from intrinsic renal failure (IRF). However, there are several instances of high FENa (>2%) in cases of PRF and low FENa (<1%) in IRF patients. In contrast, the fractional excretion of urea nitrogen (FEUN) is primarily dependent on passive forces, and many confounding variables that affect FENa have little effect on FEUN, if any. To compare FEUN with FENa, pediatric patients with acute kidney injury (AKI) were prospectively evaluated by history, physical examination, and obtaining appropriate laboratory data during a 1-year interval. Diagnosis of PRF or IRF was made in each patient, and renal failure indices were compared between two groups using chi-square and t test, as appropriate. Probability value (P value) <0.05 was considered significant. Receiver operating characteristic (ROC) plots for FEUN and FENa were drawn to compare the discriminative power of each index. Forty-three patients were enrolled in the study. There were 27 patients in the PRF and 16 in the IRF group. FENa was 2+/-0.4 in PRF and 4.5+/- 1% in IRF patients (P<0.05), and low FENa (<1%) was only seen in 44.4% of PRF patients, which was not statistically different from those with IRF (P>0.05). FEUN was 23.6+/- 4.9% in PRF and 41.6+/-4.8% in IRF patients (P<0.05), and low FEUN (<35%) was seen in 77.8% of the PRF group (P<0.05). Cutoff values of 30% and 1.6% were reached for FEUN and FENa, respectively. In conclusion, FEUN <35% had higher sensitivity and specificity than FENa <1% for differentiation of PRF from IRF.