Oliguria is defined as a 24-hour urine output of less than 400 ml. In a normal person, the 24-hour urine output is 1000-2000 ml. Both anuria and oliguria are caused by a decrease in renal excretion. The cause of decreased urine output may be pre-renal, renal or post-renal. It is important to confirm the presence of ureteral or urethral obstruction causing oliguria or anuria. Anuria is defined as a persistent absence of urine with a volume of less than 100 ml in 24 hours and is seen in organic kidney injury, manifesting as azotemia and uremia. Urine volume of less than 400ml in 24 hours is oliguria. Sudden decrease in urine volume may occur as acute kidney injury with oliguria, which should be given full attention.