Renal expression of neutrophil gelatinase-associated lipocalin (NGAL) increases dramatically after renal ischemia or exposure to nephrotoxic agents. This is reflected by the rapid rise in urinary NGAL reported in children who developed acute renal failure (ARF) after cardiac surgery. The aim of the present study was to ascertain whether urine or plasma NGAL levels, determined by a recently developed ELISA, are useful for diagnosing renal injury leading to ARF in unselected critically ill patients admitted to intensive care.