Background: Computed tomography (CT) assessment of air trapping has been considered useful as a measure of small airway disease. Mean lung density (MLD) and the percentage of the lung field occupied by low attenuation area (LAA%) can be evaluated automatically, and their expiratory/inspiratory (E/I) ratios correlate with asthma severity and spirometry parameters. However, mosaic attenuation, another indicator of air trapping, has been assessed visually, and its functional relevance remains controversial. Objectives: This retrospective study was conducted to correlate mosaic attenuation, which was assessed visually and automatically, and the E/I ratios of MLD and LAA% (defined as areas (Source: Respiration)