To investigate the condition of peripheral eosinophils in patients with bronchial hyperresponsiveness, we examined density distribution profiles and ultrastructure of peripheral eosinophils obtained before and after exercise challenge test. Following exercise challenge test, the number of hypodense eosinophils (<1.0825 g/ml) was significantly higher in the EIA-positive than in the EIA-negative subjects. Regarding ultrastructural analysis, the ratio of the area of specific granules to the area of cytoplasm (SG/C) per cell was significantly higher in the EIA-negative than in the EIA-positive patients. The ratio of the degranulated area to the area of cytoplasm (D/C) per cell was significantly higher in the EIA-positive than in the EIA-negative subjects. These data suggested that activation of eosinophils in the patients with bronchial hyperresponsiveness occurs during the preinflammatory phase.