A multiple-breath washout technique was used to assess residual ventilation heterogeneity in the conductive and acinar lung zones of asthmatic patients after maximal β₂-agonist reversibility. Reversibility was assessed in 13 patients on two separate visits corresponding to a different baseline condition in terms of forced expiratory volume in 1 s [FEV₁; average FEV₁ over 2 visits: 92 ± 21% of predicted (SE)]. On the visit corresponding to each patient's best baseline, 400 μg salbutamol led to normal acinar ventilation heterogeneity, normal FEV₁, and normal peak expiratory flow; i.e., none was significantly different from that obtained in 13 matched controls. By contrast, conductive ventilation heterogeneity and forced expiratory flow after exhalation of 75% forced vital capacity remained significantly different from controls ( P ≤ 0.005 on both indexes). In addition, the degree of postdilation conductive ventilation heterogeneity was similar to what was previously obtained in asthmatic individuals with a 19% lower baseline FEV₁and twofold larger acinar ventilation heterogeneity (Verbanck S, Schuermans D, Noppen M, Van Muylem A, Paiva M, and Vincken W. Am J Respir Crit Care Med 159: 1545–1550, 1999). We conclude that, even in the mildest forms of asthma, the most consistent pattern of non-β₂-agonist-reversible ventilatory heterogeneity is in the conductive lung zone, most probably in the small conductive airways.