Prerenal failure is traditionally accompanied by oliguria and represents the normal renal adaptation to retain salt and water and correct the prerenal state. Nonoliguria occurring in the setting of acute renal failure usually represents acute tubular necrosis (ATN) since the kidney has lost its ability to extract salt and water. We report nine cases of patients with acute renal failure occurring in the setting of impaired systemic hemodynamic states and yet who were nonoliguric without strong evidence for ATN. The common defect in these subjects with "polyuric prerenal failure" was a blunted urinary concentrating ability. Polyuria and renal failure occurring despite evidence for impaired systemic hemodynamics may not necessarily be ATN, may still be prerenal, and should be recognizable and promptly reversible if treated appropriately.