In this study, a total of 60 patients with acute bronchitis, 71 patients with bronchial asthma and 20 healthy volunteers were serologically and bacteriologically analyzed to investigate whether Chlamydia pneumoniae infection is associated with the onset and the exacerbation with acute bronchitis and bronchial asthma. Antibody titers to Chlamydia pneumoniae were also measured and compared by ELISA method. The antibody-positive rate in the patients with acute bronchitis (88.4%) was significantly higher than that in the patients with bronchial asthma (73.3%) or that in the healthy volunteers (60%). And the levels of the IgA antibody in the patients with acute bronchitis were significantly higher than those in the patients with bronchial asthma or those in the healthy volunteers. The rate of acute C. pneumoniae infection in the patients with acute bronchitis (20%) did not show significantly differences compared with that in the patients with bronchial asthma (15.5%) or that in the healthy volunteers (10%). The cases of acute C. pneumoniae infection had both as a single etiologic agent and as a mixed infection, most often with Streptococcus pneumoniae. Therefore, we demonstrated that the acute C. pneumoniae infection may be associated with the onset and the exacerbation in acute bronchitis and bronchial asthma.