Significant renal dysfunction is common in patients hospitalized for heart failure and carries a grim prognosis. Patients with heart failure who have or develop renal dysfunction while being treated for heart failure are said to have the cardiorenal syndrome. The Acute Decompensated Heart Failure National Registry (ADHERE) database, which enrolled nonselected patients admitted to the hospital for acute decompensated heart failure (ADHF), was used to determine the causes for this renal dysfunction and whether treatment can optimize outcomes. Results show that the average patient admitted for ADHF is older than those typically enrolled in clinical trials and has at least moderate kidney damage, with significantly impaired glomerular filtration rates. Renal dysfunction in patients with heart failure is complex and often multifactorial in origin, but the syndrome may be reversible in some patients. Reduction of angiotensin II levels with angiotensin-converting enzyme (ACE) inhibitors may prevent glomerular hyperfiltration and ultimately preserve renal function; however, patients who are volume-depleted may be especially sensitive to ACE inhibitor-induced efferent arteriolar dilation, so ACE inhibitor therapy in patients with renal dysfunction should be initiated when the patient is volume replete. In conclusion, impaired renal function is common in heart failure patients and may be a key cause of the cascade involving fluid retention, decompensation, and eventual hospital admission. Future pharmacologic research should focus on therapies aimed at maintaining or improving renal function in heart failure patients to reduce the high mortality associated with the cardiorenal syndrome.