A decrease in urine output is the most visible sign of acute renal failure (ARF) in all age groups, particularly younger children. Oliguria occurs when the urine output in an infant is less than 0.5 mL/kg per hour for 24 hours or is less than 500 mL/1.73 m[2] per day in older children. Anuria is defined as absence of any urine output. An important point to remember is that healthy newborns may have no urine output for 24 hours after birth.

Oliguria is much more common than anuria, and, if not treated appropriately, the patients may become anuric, which may result in serious renal damage that requires specialized care. Oliguria or anuria may be the outcome of a renal response to intravascular circulatory volume depletion or a sudden drop in the blood pressure, also called prerenal ARF. Oliguria or anuria may also occur as the result of intrinsic renal damage, or renal ARF. Rarely, obstruction to the flow of urine, or postrenal ARF, may result in oliguria or anuria.