The fixed ratio criterion of forced expiratory volume in 1 s/forced vital capacity <0.70 for diagnosing airway obstruction may overdiagnose the condition, particularly in the elderly, so the lower limit of normal (LLN) is recommended as the most appropriate criterion. Our aim was to compare LLN versus fixed ratio on the prevalence of chronic obstructive pulmonary disease (COPD) and examine the association between respiratory symptoms and airway obstruction defined by LLN and fixed ratio. 12 449 twins aged 40–80 years participated in a nationwide survey using the Danish Twin Registry. They completed a questionnaire, underwent clinical examination and recorded prebronchodilator spirometry. Individuals with self-reported asthma were excluded. Clinical COPD was defined by respiratory symptoms together with airway obstruction. 10 329 individuals were included, with a mean±sd age of 58.4±9.6 years and mean body mass index of 26.6±4.4 kg·m−2; 20% were current smokers, 37% former smokers and 43% never-smokers; and 48% were male. The prevalence of LLN airway obstruction (LLN-AO) and fixed ratio airway obstruction (FR-AO) was 5.6% and 18.0%, respectively (p<0.001). Overall, 26% reported current respiratory symptoms, but 50% of those with LLN-AO had respiratory symptoms compared to 39% with FR-AO, p<0.001. The prevalence of clinical LLN-COPD and fixed ratio COPD was 2.6% and 6.3%, respectively (p<0.001). Individuals with LLN-AO had a significantly higher probability of reporting respiratory symptoms compared with both healthy individuals and FR-AO when adjusted for sex, age and ever-smoking. The use of fixed ratio more than doubled the prevalence of clinical COPD compared with LLN, this being more pronounced with increased age, and identified subjects with a lower prevalence of respiratory symptoms than LLN-AO.