The geometry of an enlarged reconstructed human acinus (i.e., a terminal bronchiole and distal airways and air spaces) was studied. Alveoli were categorized in six shapes: three-fourths of a spheroid, a slightly truncated cone, one-fourth of a spheroid, a cylindroid with a hemispherical bottom, a shallow cylindroid with a flat bottom, and a truncated deep ellipsoid. Sacs were usually either hemispheroids or shallow truncated cones. Ducts of eight generations were spheroid and gradually decreased in diameter (D) and length (L) as the generation number (z) increased. Considering the terminal bronchiole as the 15th generation and using Weibel's data for the first 10 generations, the dimensions, in mm, for z of 1–10 and 10–26 were reasonably described by D-z = 12e-(0.27–0.005z)z and L-z = 25e-0.187z. The predicted volume of the acinus at three-fourths total lung volume was 182.8 mm3, a volume equivalent to that of a sphere 7.04 mm in diameter. The reconstruction demonstrated a great increase in respiratory bronchiolar and ductal cross-sectional area and alveolar surface area, considerably more rapid than predicted by Weibel's model A.