The FENa test, a determination of the excreted fraction of the filtered sodium, was performed in patients in the oliguric phase of acute renal failure. Patients with prerenal azotemia had an FENa of less than 1, and patients with acute tubular necrosis had an FENa of more than 3 (P less than .001). This simple test clearly differentiates between these two conditions and, thus, is of considerable clinical value.