ABSTRACT Background The effect of short-term corticosteroid treatment for airway inflammation in asthma is not well known. To investigate this effect, we analyzed bronchial mucosal biopsy samples. Methods We obtained endobronchial biopsy specimens from subjects with asthma with or without corticosteroid pretreatment. Samples obtained were analyzed by immunohistochemical staining (CD4, CD25 and EG2). Results Activated eosinophils were significantly reduced in the corticosteroid-premedicated group (CS +) compared with the unpremedicated (CS-) group. In contrast, no significant reduction was observed in CD4-positive T lymphocytes between the two groups. The number of CD25-positive T lymphocytes was more suppressed in the premedicated than in the unpremedicated group. Conclusions The effect of corticosteroid on eosinophils in bronchial mucosa develops within a few days. However, for a clinically significant effect, long-term corticosteroid treatment may be needed to inactivate and displace T lymphocytes from the airway mucosa.