Two hundred fifty-five determinations of the protamine sulfate test, the staphylococcal clumping test, and the tanned red blood cell hemagglutination inhibition immunoassay were performed on 30 patients undergoing cardiovascular surgery. The level of serum fibrin-fibrinogen degradation products rose in all patients and reached the highest levels between the fifth and ninth postoperative day. After the first postoperative day, a fibrin-fibrinogen degradation product increase of greater than 10μg/ml within 24 to 48 hours was associated with clinically recognized thrombotic complications in only two out of five patients. Isolated levels of serum fibrin-fibrinogen degradation products as high as 67μg/ml could not be correlated with clinically evident thrombotic disease or serious complications of surgery. Increased excretion of urinary fibrin-fibrinogen degradation products was associated with, but not predictive of, acute ischemic renal failure.