In contrast to the increasing incidence of allergic disease in modern civilization, the pathophysiologic mechanisms underlying its development are not fully understood. Recently, there have been several lines of evidence that cytokines may play important roles in the development of allergic disease. However, the possibility that cytokines may play important roles in the pathophysiology of allergic disease has been suggested mainly by the in vitro investigation on the action of individual cytokine, and the information regarding the pattern of cytokine expression in vivo in allergy, especially in human nasal allergy, is still limited. Furthermore, it is not clear whether T-helper cell dichotomy suggested by Mossmann et al in mice, in which Th₂ lymphocytes secrete IL-4, IL-5, IL-6 and IL-10 which are closely related to the development of allergic disease and Th₁ lymphocytes secrete IFN-γ and TNF-β, can be applied in the human immune system. If this T-helper cell dichotomy can be applied in human and if the expression of Th₂ type cytokines are increased in contrast to Th₁ type cytokine in allergic nasal mucosa, it can suggest not only that the cytokines may play important roles in the pathophysiology of allergic disease in vivo but allergen specific T-helper cells may play a more crucial role in the development of allergic disease that has been thought. In order to further understand the role of cytokines in the pathophysiology of allergic disease in vivo in human, this study was performed to define the pattern of expression of cytokine mRNA in the nasal mucosa of patients with perennial allergic rhinitis by performing RT-PCR (reverse transcription-polymerase chain reaction) for IL-4, IL-5 (Th₂ cytokine) and INFγ-(Th₁ cytokine), which revealed increased expression of IL-4 and IL-5 mRNA in the nasal mucosa of patients with perennial allergic rhinitis during natural allergen exposure.