To investigate age-related changes in the shape of trachea, normal male volunteers (n = 83, mean +/- SD: 47.7 +/- 20.2 years old) underwent inspiratory CT scans at full inspiration and lung function tests. Subjects who showed VC < 80% predicted or FEV1 < 80% predicted on lung function tests were excluded. The CT data, which is located at 2.0 cm above the aortic arch, were transferred to a personal computer. The tracheal area (St) and two parameters, Tracheal index (Ti) and Circularity (Ci) indicating the shape of the trachea, were automatically calculated. Ti was defined the ratio of the coronal to the sagittal diameter of the trachea, and the Ci (Ci = 4piS/L2, S: tracheal area, L: tracheal perimeter) was used to indicate the roundness of the trachea. A Ci value of less than 1 indicated the distortion of the roundness. Both St and St/BSA (body surface area) showed a significant correlation with age (r = 0.37, r = 0.52; p = 0.0006, p < 0.0001). Ti was not correlated with age (r = -0.20; p = 0.0697), whereas Ci was significantly correlated with age (r = -0.32; p = 0.00364). There were measurable age related changes of the trachea both in the area and the shape. Aging results in the increased tracheal area and a distortion of the roundness.