The term "cardiorenal syndrome" has been applied to the presence or development of a renal dysfunction in heart failure patients. Renal function that worsens during hospitalization is a major precipitant of decompensation and cause for admissions in heart failure patients and is a more important predictor of adverse outcome than baseline renal function. Despite growing recognition of the frequent presentation of this combined cardiac and renal dysfunction, its underlying pathophysiology has not been well described and its management remains even less well understood.