When known causes of cough, such as various infections, cancer, foreign body aspiration, cystic fibrosis, alveolitis, asthma, chronic obstructive pulmonary disease, medication with angiotensin-converting enzyme (ACE) inhibitor, gastro-oesophageal reflux disease (GERD), or post-nasal drip syndrome have been excluded, a group of patients with unexplained cough still remains. Here, ACE is linked to chronic obstructive pulmonary disease.