The allergic response to common environmental agents (allergens) has been regarded as an important mechanism in the development of airway inflammation of patients with asthma. However, allergic sensitization cannot be detected in a significant number of adult patients with asthma. The etiologic mechanism responsible for nonallergic asthma has not yet been identified. The idea of a possible involvement of autoimmunity in the pathogenesis of nonallergic asthma has been proposed by earlier studies. To test for the possible presence of an autoimmune response to bronchial epithelial cell antigens in nonallergic asthma, we examined circulating autoantibodies to cultured human bronchial epithelial cells (BEAS-2B) in sera from patients with nonallergic asthma by immunoblot analysis. IgG autoantibodies to the 49-kD bronchial epithelial cell antigen were detected in 10 of 23 patients with nonallergic asthma (43%), 3 of 27 patients with allergic asthma (11%), 2 of 20 patients with systemic lupus erythematosus (10%), and 3 of 34 healthy volunteers (9%) (p < 0.005). The 49-kD auto-antigen was purified and identified as cytokeratin 18 by amino acid sequencing. In this study, we identified cytokeratin 18 as a bronchial epithelial autoantigen associated with nonallergic asthma. Further studies are needed to determine the significance of autoimmunity in nonallergic asthma.