Levels of serum elastase 1 in a variety of respiratory diseases were studied. In patients with pulmonary emphysema, pulmonary fibrosis, bronchial asthma, or pulmonary infections, including pneumonia and pulmonary tuberculosis, serum elastase 1 levels were greater than those of an age-matched control group. In lung cancer patients, however, the serum elastase 1 level was within normal limits. Although alpha 1-antitrypsin levels were significantly higher in patients with pulmonary infections and lung cancer than in the normal group, they were within normal limits in patients with pulmonary emphysema, pulmonary fibrosis, and bronchial asthma. Alpha 2-macroglobulin levels were slightly increased in patients with pulmonary emphysema and pneumonia. These results suggest that the increases in serum elastase 1 levels in these respiratory diseases may be mainly caused by an imbalance of elastase/antielastase system in the lung tissue and the bloodstream.