Background Small airway dysfunction is a common but neglected respiratory abnormality. Little is known about its prevalence, risk factors, and prognostic factors in China or anywhere else in the world. We aimed to estimate the prevalence of small airway dysfunction using spirometry before and after bronchodilation, both overall and in specific population subgroups; assess its association with a range of lifestyle and environmental factors (particularly smoking); and estimate the burden of small airway dysfunction in China. Methods From June, 2012, to May, 2015, the nationally representative China Pulmonary Health study invited 57 779 adults to participate using a multistage stratified sampling method from ten provinces (or equivalent), and 50 479 patients with valid lung function testing results were included in the analysis. We diagnosed small airway dysfunction on the basis of at least two of the following three indicators of lung function being less than 65% of predicted: maximal mid-expiratory flow, forced expiratory flow (FEF) 50%, and FEF 75%. Small airway dysfunction was further categorised into pre-small airway dysfunction (defined as having normal FEV, and FEV,/forced vital capacity [FVC] ratio before bronchodilator inhalation), and post-small airway dysfunction (defined as having normal FEV1 and FEV1/FVC ratio both before and after bronchodilator inhalation). Logistic regression yielded adjusted odds ratios (ORs) for small airway dysfunction associated with smoking and other lifestyle and environmental factors. We further estimated the total number of cases of small airway dysfunction in China by applying present study findings to national census data. Findings Overall the prevalence of small airway dysfunction was 43.5% (95% CI 40.7-46.3), pre-small airway dysfunction was 25.5% (23.6-27.5), and post-small airway dysfunction was 11.3% (10.3-12.5). After multifactor regression analysis, the risk of small airway dysfunction was significantly associated with age, gender, urbanisation, education level, cigarette smoking, passive smoking, biomass use, exposure to high particulate matter with a diameter less than 2.5 mu m (PM2.5) concentrations, history of chronic cough during childhood, history of childhood pneumonia or bronchitis, parental history of respiratory diseases, and increase of body-mass index (BMI) by 5 kg/m(2). The ORs for small airway dysfunction and pre-small airway dysfunction were similar, whereas larger effect sizes were generally seen for post-small airway dysfunction than for either small airway dysfunction or presmall airway dysfunction. For post-small airway dysfunction, cigarette smoking, exposure to PM2.5, and increase of BMI by S kg/m(2) were significantly associated with increased risk, among preventable risk factors. There was also a dose-response association between cigarette smoking and post-small airway dysfunction among men, but not among women. We estimate that, in 2015, 426 (95% CI 411-468) million adults had small airway dysfunction, 253 (238-278) million had pre-small airway dysfunction, and 111 (104-126) million had post-small airway dysfunction in China. Interpretation In China, spirometry-defined small airway dysfunction is highly prevalent, with cigarette smoking being a major modifiable risk factor, along with PM2.5 exposure and increase of BMI by 5 kg/m(2). Our findings emphasise the urgent need to develop and implement effective primary and secondary prevention strategies to reduce the burden of this condition in the general population. Copyright (C) 2020 Elsevier Ltd. All rights reserved.