Effective airway dimensions (EADs) were determined in vivo by aerosol-derived airway morphometry as a function of volumetric lung depth (VLD) to identify and characterize, noninvasively, the caliber of the transitional bronchiole region of the human lung and to compare the EADs by age, gender, and disease. By logarithmically plotting EAD vs. VLD, two distinct regions of the lung emerged that were identified by characteristic line slopes. The intersection of proximal and distal segments was defined as VLD_trans and associated EAD_trans. In our normal subjects (n = 20), VLD_trans [345 ± 83 (SD) ml] correlated significantly with anatomic dead space (224 ± 34 ml) and end of phase II of single-breath nitrogen washout (360 ± 53 ml). The corresponding EAD_trans was 0.42 ± 0.07 mm, in agreement with other ex vivo measurements of the transitional bronchioles. VLD_trans was smaller (216 ± 64 ml) and EAD_trans was larger (0.83 ± 0.04 mm) in our patients with chronic obstructive pulmonary disease (n = 13). VLD_trans increased with age for children (age 8-18 yr; P = 0.006, n = 26) and with total lung capacity for age 8-81 yr (P < 0.001, n = 61). This study extends the usefulness of aerosol-derived airway morphometry to in vivo measurements of the transitional bronchioles.