Normal data for tracheal shape and dimensions were obtained by the investigation of a group of 90 men and women aged 50 years or over with normal pulmonary function. Significant differences were found between the sexes in regard to tracheal shape and cross sectional area. Another group of 90 men and women with pulmonary function data indicative of severe chronic obstructive lung disease was also investigated and the tracheal findings were compared with those of the control group. It was demonstrated that patients with chronic cough and chronic bronchitis exhibit a tendency to intrathoracic coronal narrowing and sagittal widening i.c. "saber-sheath" deformity. Mechanisms responsible for this alteration in tracheal shape are discussed. No definite correlation was demonstrable between age, total lung capacity, residual volume and tracheal shape. The relatively unknown, but fairly characteristic radiological appearance of the "saber-sheath" trachea is described and the importance of routine X-ray of the trachea in patients with chronic obstructive lung disease is stressed.