The clinical significance of low attenuation area <-950 HU of the lungs on high - resolution computed tomography (HRCT) was evaluated in 80 patients with asthma in relation to patient age, CT number, pulmonary function including % residusal volume (% RV) and % DLco, and generation of leukotrienes B4 and C4 by leucocytes. 1. The % LAA (<- 950 HU) of the lungs showed a tendency to increase significantly with aging; the % LAA was significantly larger in elderly patients over the age of 60 years than in those under the age of 49 years. The CT number was also significantlyincreased with aging. 2. The values of % FVC, % FEV1.0, and FEV1.0% were significantly decreased in elderly patients over age 70 compared with the values in those under age 49. 3. The value of % residual volume (% RV) tended to increase significantly with aging; the value in patients over the age of 70 years was significantly larger in those between the ages of 50 and 59 (p