In obese people, the presence of adipose tissue around the ribcage and abdomen and in the visceral cavity loads the chest wall and reduces functional residual capacity (FRC). The reduction in FRC and in expiratory reserve volume (ERV) is detectable even at a modest increase in weight. However, obesity has little direct effect on airway caliber. Spirometric variables decrease in proportion to lung volumes, but are rarely below the normal range even in the extremely obese, while reductions in expiratory flows and increases in airway resistance are largely normalised by adjusting for lung volumes. Nevertheless, the reduction in FRC has consequences for other aspects of lung function. A low FRC increases the risk of both expiratory flow limitation and airway closure. Marked reductions in ERV may lead to abnormalities in ventilation distribution with closure of airways in the dependent zones of the lung, and ventilation perfusion inequalities. Greater airway closure during tidal breathing is associated with lower arterial oxygen saturation in some subjects, even though DLCO is normal or increased in the obese. Bronchoconstriction has the potential to enhance the effects of obesity on airway closure and thus on ventilation distribution. Thus obesity has effects on lung function that can reduce respiratory well-being, even in the absence of specific respiratory disease, and may also exaggerate the effects of existing airway disease.