The concern is raised that our “findings may be explained by an underestimation of lung volumes using the CT scan method resulting from submaximal inspiration in the supine position.” We did discuss the potential influence of the supine position on lung volume measurements. As we reported, at H1 (where CT imaging was performed during spirometric monitoring) we compared upright vs supine vital capacities, and the mean difference was approximately 80 mL (upper 95% CI <200 mL). Two other pieces of evidence suggest that supine position alone does not explain the observed differences in TLC by technique. The first is lack of by-technique difference in TLC among healthy individuals. Thus, for the supine position to explain our data, it would require that TLC from supine position be less than that from a seated position only in those with obstruction and that the magnitude of the difference increase with severity of obstruction. We do not know of any evidence that this is the case, nor does Dr Stănescu cite any such evidence in defense of his criticism. The second is the very close correspondence between He dilution and CT scan lung volume among subjects with COPD (mean difference = 150 mL). To argue that our results in those with obstruction are explained in one instance by underestimation of TLC by CT scan, measured with subjects supine, and in another instance underestimation by He, measured with subjects seated, would require the highly unlikely coincidence that the two underestimations are equal and independent of the severity of the obstruction. Dr Stănescu does not provide us with a hypothesis or data supporting the justification for his suggestion that this coincidence exists.