PURPOSE: We applied spiral CT to evaluate the dynamic changes of regional ventilation of the lung in normal subjects and abnormal patients. MATERIALS AND METHODS: This study includes normal subjects (n:5) and patients with chronic obstructive pulmonary disease (n=4), small air-way disease (n=3), diffuse panbronchiolitis (n=4), and tracheobronchial tuberculosis (n=2). Time-continuous scan data at a fixed level during forced vital capacity maneuver (10--12 seconds) were obtained and images were reconstructed retrospectively by using 0.67 second scan data per image. The reconstructed images were displayed in a cine mode. Time-density curves were plotted and were correlated with clinical diagnosis. RESULTS: In normal subjects, mean attenuation difference between full inspiration and full expiration was 145. 8HU and mean time interval between 20% expiration and 80% expiration was 2.04 seconds. In chronic obstructive lung disease, mean attenuation difference between full inspiration and full expiration was 21.2HU and mean time interval between 20% expiration and 80% expiration was 3.63 seconds. In small air-way disease, mosaic-pattern hyperlucency and normal portion of lung showed mean attenuation differences between 20% expiration and 80% expiration to be 49.8HU and 167.0HU, respectively. In diffuse panbronchiolitis, centrilobular region and normal portion of lung showed mean attenuation differences between 20% expiration and 80% expiration to be 35.4HU and 79.3HU, respectively. CONCLUSION: Respiratory dynamic CT is an updated technique which enable imaging of the functional status of the lung parenchyma. It may be useful in differentiation and quantitation of variable obstructive lung diseases.