Most studies agree that post-nasal drip syndrome (PNDS), asthma, gastroesophageal reflux disease (GORD), and laryngopharyngeal reflux (LPR) are the commonest causes of chronic cough in the immunocompetent, non-smoking patient who is not taking an angiotensin-converting enzyme inhibitor. No diagnostic test has been found to define those who are said to have PNDS other than a response to a first-generation antihistamine. Examining the available evidence suggests that mechanical stimulation of the pharynx by mucus is not an adequate theory for the production of cough. Inflammatory mediators in the lower airways are raised in PNDS, cough variant asthma and GORD, and the theory that an inflammatory process is affecting 'one airway' is a plausible one. Nasal disease is more likely to result in cough from the co-existing involvement of the lower airways through an as yet undefined pathway, and eosinophil and mast cell mediation appear a likely mechanism.