Given the unified airway theory that causes of chronic cough frequently coexist, we identified the frequency of coexistence of possible causes. When possible causes were classified by current smoking status, UACS, COPD, asthma, and chest radiographic abnormality, 50.3% ± 4.5% of those with chronic cough had coexisting possible causes (Fig. 2). Additionally, 12.7% ± 3.2% of those with chronic cough were suspected to have 3 or more possible causes. However, no possible causes were noted in 14.7% ± 3.1% of subjects. The most highly overlapped conditions were UACS in subjects with asthma (66.9% ± 10.8%) and chest radiographic abnormalities in those with COPD (9.9% ± 4.3%) or asthma (9.4% ± 6.3%) (Table 3).