Background: Assessment of low attenuation areas and lung densitometry on high resolution computed tomography (HRCT) have been reported in studies of patients with chronic obstructive pulmonary disease. Objective : The purpose of this study was to examine if HRCT could separate asthmatic patients from normal control subjects and patients with emphysema. Methods : Subjects were divided into three groups : 24 patients with bronchial asthma, 23 patients with pulmonary emphysema and 15 normal controls. HRCT scans of the lung were performed at three different levels at both end inspiration and expiration in all patients and control subjects. Results : The mean CT number of three slices was significantly lower in asthmatic subjects compared with normal individuals at both end inspiration (p < 0.05) and expiration (p < 0.01). The relative area of the lung with an attenuation value lower than -950 HU (% LAA) for three slices was significantly higher in asthmatic subjects than in normal individuals at end inspiration (p < 0.05), but there was no significant difference at end expiration. The mean CT number and % LAA obtained from studies that were performed at three cm above the top of the diaphragm provided the best separation among three anatomic levels. In comparison between asthmatic and emphysema patients, both parameters were significantly different in asthmatic subjects than in those with emphysema at both end inspiration and expiration on each scan and in the total scans (p < 0.001). Conclusions : HRCT is a useful method to separate asthmatic subjects from patients with emphysema and normal subjects.