Cough is common in primary care.1 Although most coughs are short-lived and self-limiting, those that persist have an impact on the quality of life.2 By definition, isolated chronic cough lasts >8 weeks and is unexplained by chest X-ray findings.3 Smoking, angiotensin-converting enzyme inhibitor (ACEi) medication, asthma, gastro-oesophageal reflux disease (GORD) and upper airway pathology (chronic rhinosinusitis or post-nasal drip) are considered common contributing causes,4 but the relative frequency of each probably depends on the clinical setting. This evidence concerns the gene ACE and asthma.