Neonatal acute renal failure (ARF) is defined as the sudden, severe derangement of glomerular filtration and tubular function and is diagnosed when serum creatinine is greater than 1. 5 mg/dL regardless of the rate of urine output. ARF is classified as pre-renal, intrinsic renal and postnatal. In practice, two forms may coexist, e. g. pre-renal and intrinsic renal mechanisms or one form of ARF may lead to another, e. g. pre-renal or post-renal leading to intrinsic renal failure. Since the most common etiology of ARF in neonates is prerenal, the bulk of the discussion of pathophysiology will be centered on this entity. In this form of ARF, rapid correction of the underlying disturbance and restoration of circulating blood volume will improve glomerular function. Post-renal failure refers to obstruction of urine flow in both kidneys most commonly from posterior urethral valves in males.