Bronchial asthma is characterized by infiltration of the respiratory tracts by eosinophils. A wide variety of adhesion molecules expressed by eosinophils have been proposed to be involved in binding of eosinophils to the vascular endothelium and subsequent transmigration from the circulation to the airways, while little is known about CD44 expression on eosinophils. We introduced a novel staining combination with which surface markers on eosinophils could be analyzed without purification prior to staining, and examined the expression of CD44 on eosinophils. Staining of eosinophils with anti-CD 16 and anti-VLA-4 mAbs enabled us to delineate eosinophils as VLA-4^high CD 16- cells from any other leukocyte populations in the whole blood. CD44 was found to be constitutively expressed on resting eosinophils, and expression increased upon cytokine-mediated activation. In all bronchial asthma patients examined, CD44^high eosinophils were enriched in sputum relative to peripheral blood, indicating that eosinophils in sputum were more activated than those in blood. By comparing the extent of CD44 expression on blood eosinophils from poorly controlled and well-controlled asthma patients, we unexpectedly found that the density of CD44 expression is higher on blood eosinophils from the well-controlled group. Thus, the extent of CD44 expression on blood eosinophils is a novel marker indicative of the mangement of bronchial asthma. Deterioration of the asthma management with a concomitant decrease in CD44 expression on peripheral eosinophils implies that CD44 may play an important role in facilitating the transmigration of activated, CD44^high eosinophils from the circulation to the respiratory tracts.