Causes of renal failure and urinary indexes were recorded in 45 neonates with oliguria and uremia. Twenty (44%) had ischemic renal damage; nine of these infants recovered and 11 died. Eight infants (18%) had various other causes of renal insufficiency. Determination of serum BUN or creatinine concentrations did not differentiate between these groups. Fractional excretion of sodium (FENa) and renal failure index (RFI) determined on serum and first-voided urine samples were statistically different between the neonates with prerenal uremia and the neonates with ischemic renal damage, although there was overlap between the groups. A neonate with an FENa less than 2.5% and an RFI less than 2.5 is said to have prerenal uremia. An FENa greater than 2.5% or an RFI greater than 2.5 in a neonate suggests ischemic renal disease; however, some neonates with prerenal uremia may have values in this range.