Silicone rubber corrosion casts of the human lung in a state of end inspiration were used to study several specimens of the human pulmonary acinus. Four of the acini were measured in detail with respect to duct length and diameter, the number of alveoli per duct, and the branching pattern of the ducts. The acini were found to have irregular branching patterns, including dichotomous, trichotomous, and side branches. There were, on the average, eight to 12 duct generations and about 7.1 × 103 alveoli per acinus. The polygonal alveoli had an average diameter of 250 μm. The lengths and diameters of the ducts varied considerably; however, the dimensions tended to decrease in the more proximal portions of the acini. The number of alveoli per duct also varied, with an average of ten alveoli per duct. On the basis of the measurements, two models, a “surrogate path” model and an “average path” model, were used to summarize the data, with the surrogate path model being more useful for calculations such as particle desposition in the airways, and the average path model being most illustrative of the anatomical structures.