One hundred fifty of 490 patients undergoing open heart surgery had renal failure attributable to cardiopulmonary bypass. In 69, serum creatinine concentrations did not exceed 2 mg/dl and returned to normal by the fourth postoperative day. In 60 patients, serum creatinine attained levels between 2 and 5 mg/dl, oliguria did not develop, and recovery of renal function occurred within 4 to 37 days. Serum creatinine increased to levels exceeding 5 mg/dl in 21 patients, 11 of whom were oliguric. Despite dialysis, 14 of these patients died from cardiac causes or sepsis. Prolonged cardiopulmonary bypass time, hypotension, oliguria, low output syndrome, and hemoglobinemia during open heart surgery correlated with the development of renal failure postoperatively. Although severe renal failure was an uncommon complication after open heart surgery, its occurrence carried a grave prognosis.