Serum lactic acid dehydrogenase (LDH) and fibrinogen-fibrin degradation product (FDP-fdp) levels were measured in 35 patients, and LDH isozyme distributions in 11 patients with arteriographically proved pulmonary emboli. Levels of LDH tended to be maximal during the 48 hours following embolization; 30% of those measured during this interval were elevated. Distribution of serum LDH levels in these patients was similar to that in patients with congestive heart failure or pneumonia with effusion. The LDH isozyme patterns of our patients were characterized by increased LDH 3. However, they were not sufficiently characteristic to differentiate pulmonary embolization from congestive heart failure or pneumonia. The FDP-fdp levels were elevated or borderline in 20%. Single measurements of LDH, LDH isozyme, and FDP-fdp levels, performed when the diagnosis is suspected with these methods, are not reliable tests for pulmonary emboli.