The influence of lung inflation on the cross-sectional area of the trachea and main bronchi was investigated in 2 normal subjects, 2 patients with emphysema, and 1 patient with pulmonary fibrosis. Fiberoptic cinebronchoscopy was used to measure the cross-sectional area of the airways using calibrated small Teflon discs placed on the airway wall as reference. As an estimate of airway compliance at intermediate lung volumes, radial distensibility was defined as the change in cross-sectional area per change in airway pressure minus esophageal pressure divided by the cross-sectional area at functional residual capacity. Both in normal subjects and in patients with emphysema, the radial distensibility was larger in the trachea than in the main bronchi without differences between the right and left main bronchus. The area-pressure curves revealed alinearity and hysteresis in both groups with an increased distensibility in pulmonary emphysema. In a patient with pulmonary fibrosis, the distensibility was decreased by a factor of ten and the area-pressure curve was linear. These observations suggest altered static mechanics of extraparenchymal, intrathoracic airways in patients with pulmonary emphysema and fibrosis.