Renal failure without apparent cause (the hepatorenal syndrome) may develop in the course of cirrhosis of the liver. While the development of renal failure bears a poor prognosis, spontaneous recovery can occur. The data suggest that for the most part patients die in rather than of renal failure. The latter seems to be only part of a broader more fundamental disturbance. The pathogenesis of HRS is unknown, but the evidence supports an impairment of effective renal perfusion. The two major hypotheses concerning the nature of the impaired perfusion are that it is a physiologic response to alterations in the extrarenal circulation, and that there is an unidentified humoral agent(s) produced by or inadequately inactivated by or bypassing the diseased liver and causing circulatory changes in the kidney as well as in other organs. It is possible that both mechanisms are operative. Treatment is unsatisfactory and emphasis is presently best placed upon searching for more treatable causes of renal functional impairment in individual patients.