To help differentiate pulmonary embolism from other lung diseases, we measured the degradation products of fibrinogen and fibrin and soluble fibrin complexes in normal control subjects and patients with pulmonary embolism, lung cancer, pneumonia, chronic obstructive pulmonary disease, tuberculosis, asthma, and several miscellaneous disorders. A separate group of patients, who were suspected of having pulmonary embolism but had negative pulmonary angiography, were also tested. Many nonthromboembolic lung diseases frequently were associated with positive fibrinogen/fibrin degradation products or soluble fibrin complexes, but those with high positivity rates for one test tended to have low rates for the other test. Both fibrinogen/fibrin degradation products and soluble fibrin complexes were positive in 55 per cent of patients with pulmonary embolism but only in 4 per cent with nonthromboembolic conditions (P less than 0.001), in 7 per cent of patients with negative pulmonary angiography (P less than 0.001), and in none of the normal subjects (P less than 0.001). Both tests were negative in only 3 per cent of patients with pulmonary embolism but in 35 per cent of nonthromboembolic diseases (P less than 0.005), 54 per cent of those with negative pulmonary angiography (P less than 0.001), and 79 per cent of normal control subjects (P less than 0.001). The combination of fibrinogen/fibrin degradation products and soluble fibrin complexes is more valuable than either test alone in the diagnostic separation of thromboembolic from nonthromboembolic pulmonary diseases.