Comparing isovolume flows, measured at the mouth during forced expiratory manoeuvres as started from maximal or partial lung inflation, is a means of assessing the effects of deep inhalation on airway calibre. The aim of this study was to investigate whether the assessment of the effect of deep inhalation during induced bronchoconstriction is influenced by the lung volume at which it is determined and by volume differences due to thoracic gas compression that occur during forced expiratory manoeuvres. Four healthy subjects and six subjects with mild-to-moderate asthma subjects performed partial and maximal forced expiratory manoeuvres in a flow-type body plethysmograph at control and during a methacholine (MCh) inhalation challenge. Mouth flow (V1) was plotted against both the expired volume (Vmo) and the simultaneous thoracic volume measured by plethysmography (Vpl) changes (V1-Vmo loop and V1-Vpl loop, respectively). The effects of deep inhalation were quantified by determining 1) the ration of maximal to partial expiratory flows (M/P) at 30, 40 and 50% of control forced vital capacity (FVC) both on V1-Vmo loops (M/Pmo) and V1-Vpl loops (M/Ppl) at control and at MCh end-point; and 2) the slope of the linear regression of maximal vs partial expiratory flows at 30, 40 and 50% of control FVC both on V1-Vmo loops (MPst,mo) and V1-Vpl loops (MPsl,pl) over the entire challenge. At control, M/Pmo and M/Ppl were similar. At MCh end-point, M/Pmo and M/Ppl increased more than twofold (p < 0.002), with M/Pmo consistently exceeding M/Ppl (p < 0.001). In addition, both M/Pmo and M/Ppl varied inversely with lung volume (p < 0.001). By contrast, MPsl,mo and MPsl,pl were not significantly different from each other (p = 0.8), and were also similar at the different lung volumes (p = 0.6). We conclude that during induced bronchoconstriction, the bronchodilation following a deep inhalation, expressed as maximal to partial flow ratio is dependent both on lung volume and volume differences due to thoracic gas compression. The use of expired flow and volume measurements may lead to a small but systematic overestimation of the bronchodilator effect of a deep inhalation. On the contrary, maximal to partial flow slope is insensitive either to lung volume or volume differences due to thoracic gas compression and can, therefore, be fairly determined from expired flow-volume loops.