In a prospective 6 month study of 204 patients requiring cardiac operations, five (2.5 percent) developed acute renal failure (ARF) and five (2.5 percent) had documented renal dysfunction (RD). Preoperative left ventricular dysfunction and prolonged cardiopulmonary bypass (CPB) were important predictors of subsequent RD/ARF; CPB pressure per se was not. Physiological and clinical studies in 51 selected patients studied over an 18 month period documented the effectiveness of low flow, low pressure CPB in preserving postoperative renal function. Twenty-two patients with nonazotemic postoperative courses demonstrated moderate depression of cardiac function while the glomerular filtration rate (GFR) was normal (98 +/- 30 ml./min/1.73 M.2) within 24 hours of operation. Seventeen high risk patients developed AFF (65 percent mortality rate) and 12 experienced severe RD without ARF (17 percent mortality). ARF (65 percent mortality rate) and 12 experienced severe RD without ARF (17 percent mortality). Eleven patients with ARF and 11 with RD were studied in the early postoperative period; at this time, all 22 patients demonstrated RD with equivalent severe depression of cardiac and renal function. Superposition of further hemodynamic or toxic insults upon ischemic kidneys was usually necessary for ARF to occur.