Computed tomography measured the lengths, anteroposterior (AP) diameters, transverse diameters, cross-sectional areas, and contained volumes of the tracheas of 130 subjects in their first two decades. Patients below age 6 were scanned at low lung volumes. The others were scanned at or near total lung capacity. The results are shown by age and gender. There were no differences between boys and girls until age 14, when girls' tracheas stopped growing. The data suggest that male tracheas continue to enlarge (but not lengthen) for a time after growth in height ceases. Mean transverse diameters tended to be greater than mean AP diameters to the age of 6; the diameters were then nearly identical until age 18, when the AP diameters usually became slightly larger. The tracheas were nearly round in cross section, especially at high lung volumes. In individual tracheas, changes from level to level were small. These measurements should be useful in the detection of tracheal abnormalities, in problems in respiratory physiology, and in endotracheal intubation, endoscopy, and tracheostomy.