To investigate the role of eosinophilis in the mechanisms of EIA we determined the changes in the density distribution profiles and ultrastructure of peripheral eosinophils from children with asthma after exercise. The subjects were 20 asthmatic children consisting of 12 EIA-positive patients and 8 EIA-negative patients and 6 normal children. Fifteen, 30 and 60 minutes after exercise, the number of hypodense eosinophils (< 1.0825 g/ml) in the EIA-positive group was significantly higher than in the EIA-positive group. In the ultrastructural findings, before exercise the ratio of the area of specific granules to the area of cytoplasm per cell (the ratio SG/C) was significantly higher in the EIA-negative patients than in the EIA-positive patients even before exercise, and the ratio of the degranulated area to the area of cytoplasm per cell (the ratio D/C) was significantly higher in the EIA-positive group than in the EIA-negative group. Thirty minutes after exercise, the ratio D/C had clearly increased in the EIA-positive group. These results suggested that eosinophils in the EIA-positive group were already activated before exercise. It will be necessary to further study the factors activating eosinophils which play some roles in EIA.