There has been no detailed study of cough sensitivity during acute lower respiratory infection. The aim of this study was to clarify cough sensitivity in Mycoplasma pneumonia, which is a well known acute lower respiratory infection with persistent nonproductive cough. We examined cough sensitivity to inhaled capsaicin and tartaric acid in both the acute and the convalescent phases of Mycoplasma pneumonia, cell differentials in bronchoalveolar lavage fluid, and pathologic findings of transbronchoscopic bronchial biopsy specimens. Although dry cough was observed in all patients during Mycoplasma pneumonia, cough sensitivity in the acute phase [capsaicin: 19.8 (GSEM, 0.214) microM, tartaric acid: 0.26 (GSEM, 0.356) M] were not enhanced compared with those in both control subjects [capsaicin: 27.9 (GSEM, 1.24) microM, tartaric acid: 0.316 (GSEM, 0.079) M] and patients in the convalescent phase [capsaicin: 15.7 (GSEM, 0.219) microM, tartaric acid: 0.50 (GSEM, 0.326) M] when all symptoms including cough had disappeared. The percentage of lymphocytes and neutrophils in bronchoalveolar lavage fluid BALF was significantly greater than in the control subjects, and lymphocyte-dominant bronchitis was observed in biopsied specimens. We conclude that cough threshold to inhaled capsaicin or tartaric acid was not enhanced during acute Mycoplasma pneumonia with lymphocyte-predominant bronchitis. This is the first report examining cough sensitivity in patients with acute lower respiratory infection with pneumonia.