Acute renal failure (ARF) occurs in as many as 8% of neonates admitted to neonatal intensive care units. Most often, ARF is recognized because of oliguria (urinary flow rate less than 1 ml/kg per hour) although nonoliguric neonatal ARF is being detected with increasing frequency. Among urinary indices utilized to differentiate oliguric neonatal ARF from prerenal oliguria, a fractional excretion of sodium greater than 3% or a renal failure index (RFI) greater than 3 are helpful in confirming ARF. Such indices must be viewed with caution in very premature infants who may have a physiologically high sodium excretion rate and in neonates with the nonoliguric form of ARF. The mortality of oliguric neonatal renal failure may be as high as 60% in medical ARF and even higher in neonates with congenital heart disease, or with anomalies of the genitourinary system. In contrast, nonoliguric renal failure in neonates has an excellent prognosis. Long-term abnormalities in glomerular filtration rate and in renal tubular function are common in survivors of neonatal ARF.