The maximal percent low attenuation area < -950 HU (% LAA) among three anatomic lung levels on high resolution computed tomography (HRCT) was examined in patients with asthma, classified by the degree of % LAA, in relation to the mean CT number, % FVC of the predicted value, and FEV 1/FVC (FEV1%). 1 . The mean CT number was closely related to the degree of % LAA of the lungs. The mean CT number was significantly lower in patients with high % LAA (mean CT number-915.3HU, % LAA 37.6%) than in those with low % LAA (-852.9HU, 4.7%). 2. The FEV1% value was significantly lower in patients with high % LAA (47.8%) than in those with low % LAA (62.2%)(p<0.05). The % FVC value was also significantly lower in patients with high % LAA (77.1%) compared to the value in those with low % LAA (101.2%). The results suggest that a large volume of LAA<-950HU of the lungs can be observed in patients with asthma, and the % LAA is closely correlated with mean CT number and the values of FEV1% and % FVC.