To understand the role of cytokines in allergic nasal disease in humans, we designed this study to investigate the expression of cytokine mRNA in the nasal mucosa of 10 control subjects and 10 patients with perennial allergic rhinitis using a reverse transcription-polymerase chain reaction (RT-PCR) for IL-4, IL-5 (Th2 cytokines), and IFN-gamma (Th1 cytokine). Both IL-4 and IL-5 mRNAs were expressed in a significantly larger number of patients with allergic rhinitis than in control subjects. When the levels of expression of cytokines and beta-actin mRNA were quantitated, the mean density ratios (cytokine/beta-actin) for IL-4 and IL-5 were higher in patients with allergic rhinitis than in control subjects. In contrast, there were no differences in the number of subjects who showed expression of IFN-gamma mRNA, or in mean density ratios for IFN-gamma between the two groups. This study suggests that both IL-4 and IL-5 mRNAs are expressed almost universally in the nasal mucosa of patients with perennial allergic rhinitis during natural allergen exposure and also that Th2-type cytokines play a crucial role in the development of allergic nasal diseases in vivo.