The cardiorenal syndrome describes the frequent clinical situation in which therapy of a patient with acute cardiac decompensation is limited by a decline of renal function. This is frequently associated with preexisting chronic renal insufficiency. It is important to be aware of this potentially fatal constellation and perform diuretic therapy slowly and carefully in order to avoid inadequate volume depletion. This may require surveillance in an intensive care unit. Ultrafiltration is an alternative to diuretic therapy which may permit greater fluid loss when beginning therapy and better control of volume reduction once heart failure symptoms improve. The individual treatment of patients with cardiorenal syndrome requires a close cooperation between cardiologists and nephrologists.