I. Introduction The main pathological feature of asthma is infiltration of eosinophils into the airways (1-3). More important than the presence of eosinophils, however, is whether or not they are activated. Eosinophils are said to be activated when at least one of four conditions is met: (1) extracellular release of granule basic proteins: eosinophil major basic protein (MBP), eosinophil cationic protein (ECP), eosinophil-derived neurotoxin/eosinophil protein-X (EDN/EPX), and eosinophil peroxidase (EPO) (4); (2) production of lipid mediators: plateletactivating factor (PAF) and leukotriene C4 (LTC4) (4); (3) production of reactive oxygen species (superoxide anion, hydrogen peroxide, and hydroxy radical (5)); and (4) migration. Three other phenomena recognized recently are also thought to be related to eosinophil activation: (I) production of cytokines (4,6); (2) surface expression of major histocompatability complex (MHC) class II antigens (human leukocyte antigen DR [HLA-DR]) (7); and (3) expression of adhesion molecules and receptors leading to cell activation after binding to their ligands (4). Finally, because the function of low-density (hypodense) eosinophils has been shown to be upregulated (8), cell density is regarded as a marker of eosinophil activation.