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Fractional excretion of filtered sodium (FENa) is typically elevated (greater than 3%) in acute tubular necrosis and decreased (less than 1%) with volume depletion. We describe a patient who developed acute tubular necrosis with a FENa of 13%. Four weeks later, he was still oliguric but had also become volume-depleted. FENa was 0.4% to 0.8%, considerably lower than FENa's measured in eight nonvolume-depleted patients in acute renal failure. Vigorous intravenous fluid therapy in this patient produced a prompt increase in urine volume and improvement in renal function. We conclude the following: (1) a marked decrease in FENa in a patient with acute tubular necrosis should suggest the development of a superimposed sodium-retaining state such as volume depletion, and (2) severe volume depletion may delay or mask recovery from acute tubular necrosis.