In most normal subjects, the fractional excretion of sodium is usually less than 1 percent but may be raised with an increase in salt intake. In acutely azotemic patients, a low fractional excretion of sodium usually indicates a prerenal process that is responsive to volume repletion. However, such a low fractional excretion of sodium also can be seen with azotemia due to hepatic or cardiac failure, as well as acute glomerulonephritis, pigment nephropathy, contrast nephrotoxicity, polyuric renal failure associated with burns, acute obstruction, renal transplant rejection, and occasionally non-oliguric acute renal failure, none of which is a volume-responsive process. A fractional excretion greater than 1 percent in acutely azotemic patients usually indicates intrinsic renal injury, but is consistent with volume depletion in patients receiving diuretics or in some patients with chronic renal insufficiency. Similarly, a low quotient in acute renal parenchymal injury is usually interpreted to indicate widespread tubular integrity, but is consistent with several different pathophysiologic processes. The fractional excretion of sodium must be interpreted in light of the specific clinical setting and other laboratory data to be useful in patient management.