Background: Interleukin (IL)-18 is emerging as an attractive participant in allergic rhinitis (AR). However, correlation of IL-18 with IL-18 binding protein (BP) in plasma, and expression of IL-18, IL-18BP and IL-18 receptor (R) in AR blood monocytes remain obscure.Methods: We investigated IL-18, IL-18BP and IL-18R expression in monocytes by using flow cytometric analysis, murine AR model, and quantitative real-time PCR in the present study.Results: It was found that plasma IL-18 and IL-1β in AR patients was higher than those in healthy control subjects. Free (f)IL-18 had a high correlation with IL-18BP, IL-1β and TNF-α in AR plasma. Proportion of IL-18 + monocytes was increased, whereas IL-18BP + monocytes were decreased in blood of patients with AR. It was found that Platanus pollen allergen extract provoked the elevated expression of IL-18 and IL-18R in AR blood monocytes. Dermatophagoides pteronyssinus , Artemisia sieversiana wild and Platanus pollen allergen extracts enhanced IL-18R protein and mRNA expression in primary monocytes from AR patients. Moreover, numbers of macrophages and IL-18R + macrophages in nasal lavage fluid (NLF) were increased, and levels of IL-18 in both plasma and NLF were elevated in AR mice.Conclusions: IL-18 is likely to participate in the development of AR as a causative factor, therefore, it could be a therapeutic target for AR.