From a cast of a human pulmonary arterial tree, the diameter, length, order, and end branches of all intact branches down to those 0.8 mm in diameter were measured and corrections for broken branches were made. A sample of structures smaller than these (0.8-0.1 mm) was similarly measured. The values for branches less than 0.1 mm in diameter were found by extrapolation and comparison with known data for the precapillary vessels. Therefore, data or estimates for each order of branching in the pulmonary tree were obtained and calculations of cross-sectional area, volume, and flow were made.