A prospective study of 500 consecutive patients surviving the first 24 hours following cardiac surgical procedures was undertaken to determine the prevalence, etiology and results of therapy for postoperative acute renal failure (ARF). Thirty-five patients developed either moderate or severe ARF and an additional 102 developed mild preprenal azotemia. Positive risk factors noted inthe development of postoperative renal failure included age, elevated preoperative concentrations of blood urea nitrogen (BUN), serum creatinine, and decreased 24 hour urine creatinine clearance. The duration of cardiopulmonary bypass (CPB), aortic cross-clamping, and the total duration of the operation also closely correlated with the incidence of ARF. In the early postoperative period, clinical assessment of hemodynamic change was most helpful in predicting postoperative ARF. Significant negative risk factors included type of operation performed, New York Heart Association classification, the use of preoperative diuretic therapy, and associated other chronic illnesses. During the operation itself, the lowest and mean blood pressures, flow rates on CPB and the presence of hemoglobinuria failed to correlate with subsequent ARF. The mortality rate for established ARF was extremely poor (88.8 per cent), and there were no survivors among those requiring dialysis. ARF following cardiac surgery is a highly lethal complication which arises in a setting of inadequate cardiac function and is associated with a multiple organ system failure. Therapy of this postoperative complication, therefore, appears to be better directed toward its prevention rather than treatment once established.