The significance of the low attenuation area (LAA) of the lungs on high resolution computed tomography (HRCT) has not been determined in patients with asthma. We examined the relationship between the percentage of lung area with CT numbers lower than -950HU (% LAA) and the mean CT number in 81 patients with reversible airway obstruction (asthma) and in 22 healthy subjects, as well as the relationship between maximal % LAA and various parameters of pulmonary function, smoking history, disease severity and allergic type in the asthmatic subjects. The mean CT number was obtained by scans at three anatomic levels, and maximal % LAA was the % LAA which had the largest value of those measured at the three anatomic levels of the lung. We found that: 1. The mean CT number was closely related to the maximal % LAA of the lungs in all subjects studied (r= -0.916, p