Patients with hypereosinophilia frequently suffer from eosinophil-mediated damages of the heart, lungs, skin, and other organs, while some do not. The reason(s) for this difference is not known. We observed that eosinophils from most patients with hypereosinophilia express the α-chain of the IL-2 receptor (CD25), and that IL-2 enhances platelet-activating factor-stimulated release ofeosinophil cationic protein from CD25-expressing but not from CD25-negative eosinophils. Such a "priming" effect has previously been described for eosinophil hematopoietins. These data suggest thatpatients with increased eosinophil surface CD25 expression are at higher risk of eosinophil degranulation and subsequent tissue damage when IL-2 is present at inflammatory sites.