A prospective analysis of the value of urinary diagnostic indices in ascertaining the cause of acute renal failure was undertaken. Our results show that in the setting of acute oliguria a diagnosis of potentially reversible prerenal azotemia is likely with urine osmolality > 500 mosm/kg H₂O, urine sodium concentration < 20 meq/litre, urine/plasma urea nitrogen ratio > 8, and urine/plasma creatinine ratio > 40. Conversely, a urine osmolality < 350 mosm/kg, urine sodium concentration > 40 meq/litre, urine/plasma urea nitrogen ratio < 3, and urine/plasma creatinine ratio < 20 suggest acute tubular necrosis. A significant number of oliguric patients will not have urinary indices that fall within these guidelines. In this setting, urine sodium concentration divided by the urine-to-plasma creatinine ratio (the renal failure index) and the fractional excretion of filtered sodium provide a reliable means of differentiating reversible prerenal azotemia from acute tubular necrosis.