Rationale: Symptoms and respiratory function tests may be difficult to assess and interpret in obese patients with asthma, particularly if the asthma is severe. It is unclear whether the dynamic changes which occur during bronchoconstriction differ between obese versus non-obese asthmatics. Objectives: To explore whether the changes in airway calibre and lung volumes which occur with acute bronchoconstriction are different in obese and non-obese asthmatics and whether any differences contribute to the quality and intensity of symptoms. Methods: Thirty female patients with asthma were studied. Spirometry, lung volume measurements and dyspnoea scores were obtained before and immediately following bronchoconstriction induced by methacholine, aiming to provoke a reduction in forced expiratory volume in one second of 30 percent. Measurements and Main Results: Body mass index was independently associated with changes in lung volume after adjustment for baseline airway calibre and hyper-responsiveness. Increases in functional residual capacity and decreases in inspiratory capacity were significantly greater in obese participants (p<0.001 and p=0.003 respectively). Conclusions: Changes in respiratory function, notably dynamic hyperinflation, are greater in obese individuals with bronchoconstriction. This may potentially alter the perception and assessment of asthma severity in obese patients with asthma.