Chronic obstructive pulmonary disease (COPD) is a common and often progressive inflammatory disease of the airways, alveoli and microvasculature that is both preventable and treatable. It is well established that smokers with mild airway obstruction, as spirometrically defined, represent the vast majority of patients with COPD, yet this population has not been extensively studied. An insidious preclinical course means that mild COPD is both underdiagnosed and undertreated. In this context, recent studies have confirmed that even patients with mild COPD can have extensive physiological impairment, which contributes to poor perceived health status compared with non-smoking healthy controls. This review describes the heterogeneous pathophysiology that can exist in COPD patients with only mild airway obstruction on spirometry. It exposes the compensatory adaptations that develop in such patients to ensure that the respiratory system fulfils its primary task of maintaining adequate pulmonary gas exchange for the prevailing metabolic demand. It demonstrates that adaptations such as increased inspiratory neural drive to the diaphragm due to combined effects of increased mechanical loading and chemostimulation underscore the increased dyspnoea and exercise intolerance in this population. Finally, based on available evidence, we present what we believe is a sound physiological rationale for earlier diagnosis in this population.