Nasal polyposis and asthma are inflammatory conditions affecting the airways characterized by the presence of inflammatory cells, particularly eosinophils (Eo) and varying degree of fibrosis. IL-4 is a 20-kDa glycoprotein that is a multifunctional cytokine considered to play an important role in eosinophilic inflammation. We examined the cellular source of immunoreactive IL-4 in nasal polyps (NP) as well as in the bronchial mucosa of patients with asthma.Approximately 90% of the IL-4 positive cells in bronchial tissues was not Eo; in contrast, about 80% of the IL-4 positive cells in NP tissues was Eo. Main source of IL-4 message in NP tissues were Eo accord-ing to the results of Northern blot analysis and in situ hybridization. These data demonstrate that Eo express the message of IL-4 in vivo, and that these cells are the primary source of IL-4 in chronically inflamed upper airway tissues (NP).