To aid in the diagnosis of lung diseases associated with airway obstruction and air-trapping, we use dynamic ultrafast high-resolution computed tomography (DUHRCT). We obtain a rapid series of ultrafast HRCT scans during a forced inspiratory and expiratory maneuver to demonstrate dynamic lung attenuation changes. Alteration in lung morphology can be seen coincident with changes in lung attenuation during different phases of the respiratory cycle. Lung attenuation changes can be evaluated qualitatively or quantitatively by using time-attenuation curves measured for specific regions of lung. In a normal population, we noted that the lingula is a common site of unsuspected and asymptomatic air-trapping. In subjects with airway obstruction, and in those with cystic lung disease, air-trapping is readily detected, even at the secondary pulmonary lobule level. In some of those cases with air-trapping, a paradoxical decrease in lung attenuation was noted during forced exhalation. Recent development of spiral CT scanners may allow more common usage of a similar dynamic technique. Simple paired inspiratory and expiratory scans, obtained during suspended respiration at the same level, may be sufficient for the detection of air-trapping in many patients.