The idiopathic hypereosinophilic syndrome (HES) is characterised by blood eosinophilia associated with organ involvement. Elevated numbers of blood neutrophils have been observed during episodes of active HES. However, an increased responsiveness of eosinophils to chemotactic and chemokinetic stimuli may explain the selective eosinophil infiltration of the tissue. We have studied the migratory responses of blood eosinophils and neutrophils from 9 patients with HES and from 13 healthy control subjects. Chemokinetic and chemotactic responses to factors acting on both cell types were analysed by means of a modification of the Boyden chamber technique. We found increased migratory responses of the eosinophils, but not of the neutrophils, from the patients with HES. Increased blood neutrophil counts in three of the patients did not coincide with alterations of the neutrophil migratory responses. Our finding of increased migratory responses of eosinophils from patients with HES towards non-specific chemoattractants suggests selective priming of eosinophils in this disease. Interleukin (IL)-5 has previously been shown to prime eosinophils for migratory responses, and successful anti-IL-5 therapy of patients with HES indicates an important role for this cytokine in the development of hypereosinophilia.