The aim of this study was to assess the potential role of macrophage migration inhibitory factor (MIF) in the pathogenesis of allergic rhinitis (AR). Serum MIF concentrations were measured by a specific enzyme-linked immunosorbent assay. In order to elucidate the cellular source of MIF, we performed double immunostaining of biopsy specimens of the nasal mucous membrane with markers for MIF and for inflammatory cells. The mean MIF level in sera from patients with AR was significantly higher than that in sera from healthy controls. Moreover, the levels were significantly correlated with the severity of the clinical symptoms. The majority of the MIF-positive cells at the site of allergic inflammation were eosinophils. These data suggest that MIF plays a role in the initiation and maintenance of AR. Eosinophils formed the largest population of MIF-producing cells; this finding suggests that they may be a major source of MIF at inflammatory sites in atopic disease.