The effect of IABP on renal function were studied in 41 patients who underwent cardiac operation. Eight of the 41 patients were acute myocardial infarction (AMI), 4 were unstable angina, and 29 were postoperative low cardiac output syndrome (LOS). In 12 patients with AMI and unstable angina, an IABP was inserted preoperatively. Urinary N-acetyl-beta-D-glucosaminidase (U-NAG) and fractional excretion of beta 2-microglobulin (FE beta 2M) in addition to conventional renal parameters such as creatinine clearance (Ccr) and free water clearance (CH2O) were measured before and during IABP. Patients were divided into three groups according to its responses within 6 to 12 hours of IABP. In Group I (25 cases) which showed the significant improvement of renal and cardiac function, the values of Ccr, CH2O improved to normal range within 6 hours after initiation of IABP. In Group II (4 cases) which showed improved renal functions in spite of insufficient improvement of cardiac functions, renal functions improved 12 hours late, showing 6 hours delay from Group I. In Group III (8 cases) both the renal functions as well as cardiac functions did show no improvement. Retrospective study demonstrated that the patients whose Ccr improved above 50 ml/min/1.48 m2 and CH2O reduced below -0.55 ml/min/1.48 m2 and within 12 hours after initiation of IABP were able to wean from IABP. However, the patients whose Ccr reduced less than 50 ml/min/1.48 m2 and CH2O grown worse above -0.55 ml/min/1.48 m2 for longer than 12 hours could not wean from IABP. And the patients whose U-NAG and FE beta 2M reduced less than 20 U/l and 3.0% during IABP support were able to wean from IABP. In conclusion, IABP was effective therapy for the patients with cardiogenic shock, LOS, and renal dysfunction.