Although a wide variety of disease processes can result in a failure of renal excretory function, the vast majority of cases with "acute renal failure" (ARF) are due to the syndrome of acute tubular necrosis (ATN). The syndrome is usually initiated by an acute injury to the proximal renal tubular epithelial cells by ischemic or nephrotoxic events. This is followed by progressive and often rapid increases in the concentration of blood urea nitrogen (BUN) and serum creatinine. In the average case, the failure of renal excretory function persists for 1 to 3 weeks, to be followed by recovery. Oliguria (urine volume less than 400 ml) is present in about half of the patients. The pathogenesis of the retention of nitrogenous waste in human ATN is the subject of controversy, but the balance of data in most patients suggests that the predominant mechanism is a profound secondary vasoconstriction in response to tubular cell injury. This may represent a teleologically appropriate response to prevent catastrophic losses of fluid that would occur, if the normally high rates of glomerular filtration continued, in the face of reduced tubular reabsorptive capacity. The mechanisms by which the tubular cell injury is communicated to the vasculature, and the mediators of the hemodynamic changes, remain to be established. The differential diagnosis in a patient with ARF, usually involves exclusion of an obstruction to the urinary tract as an initial step. The next step is to differentiate the patients with ATN from those who have renal hypoperfusion in response to events in the systemic circulation, but who otherwise have functionally and structurally intact kidneys, i.e., prerenal ARF. The kidneys of patients with prerenal ARF exhibit the normal renal response to an acute reduction in renal blood flow and glomerular filtration rate (GFR). This consists of avid reabsorption of the filtered salt and H2O, so that a small amount of concentrated and NaCl-poor urine is elaborated. The tubular cell injury in ATN syndromes prevents this response from maximally occurring, so that the urine is isosmotic and relatively rich in NaCl.