To prospectively investigate the incidence and clinical findings of "postinfectious cough" among adult patients with prolonged cough, enrolled from July 2006 to June 2008, we studied the serum antibodies of Mycoplasma pneumoniae, Chlamydophila pneumoniae and Bordetella pertussis in those who complained of cough lasting 3-8 weeks but with no abnormalities on their chest X-ray films. Mycoplasma pneumoniae bronchitis, Chlamydophila pneumoniae bronchitis, and pertussis were diagnosed based on serological criteria in 5.5%, 4.1%, 9.2% of the cases, respectively. Postinfectious cough including "post-common cold cough" comprised 39.4% of all prolonged cough cases. The above diseases lacked specific clinical findings, making it difficult to differentiate between diseases causing postinfectious cough. Postinfectious cough is thought to be a common clinical entity, however further definitions of the diagnostic criteria and rapid diagnostic procedures are desirable, first to prevent familial transmission, and secondly to differentiate from allergic diseases that cause chronic cough.