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Background: We have recently reported a strong association between organ specific autoimmune disease and idiopathic chronic cough and have suggested that cough may be caused by airway inflammation secondary to aberrant homing of activated lymphocytes to the lung. An immunopathological study was undertaken to test the hypothesis that idiopathic chronic cough is associated with lymphocytic airway inflammation. Methods: Bronchoscopy, bronchial biopsies, bronchoalveolar lavage (BAL), and peripheral blood and BAL flow cytometry were performed in 19 patients with idiopathic chronic cough, 14 with explained chronic cough, and 11 normal subjects. Results: Organ specific autoimmune disease or positive autoantibodies were present in eight of the 19 patients with idiopathic cough, in one of the 14 patients with explained cough, and in one of the 11 normal subjects. Median BAL fluid differential lymphyocyte counts were significantly higher in patients with idiopathic cough (10.0%) than in normal subjects (6.3%, 95 % confidence interval of difference 1.5 to 11.9, p = 0.01) or patients with explained cough (5.2%, 95 % CI of difference 2.0 to 10.4, p = 0.001). There were no differences in bronchial biopsy T lymphocyte counts between the groups. The mean (SE) proportion of CD3+ peripheral blood mononuclear cells expressing CD4 was significantly higher in normal subjects tha