In nasal biopsies from 17 adult patients with seasonal allergic rhinitis and from 10 healthy controls, cytokines were analyzed by reverse-transcriptase polymerase chain reaction (RT-PCR). The time-course study during winter included repeated local allergen provocation with subsequent nasal biopsies as well as biopsies taken during pollen season. The RT-PCR for CD44 yielded positive bands in 65 of 71 cases, in which cases mRNA for interleukins 2, 4, and 5 (IL-2, IL-4, and IL-5) were thus investigated by means of seminested PCR. IL-4 mRNA was found almost exclusively in the allergic patients. During provocation a significant increase in IL-4 was noticed compared with controls (p = 0.043). Equally, during the natural pollen season, IL-4 mRNA expression was significantly higher in patientsnotusing nasal corticosteroids compared with those who did (p = 0.011). No differences in IL-2 or IL-5 were observed between the groups. These findings also indicate, together with earlier observations of T-cell activation, a phenotype switch toward T-helper 2 (Th2) cells, and the accumulation (homing) of these T cells in the nasal mucosa, that T cells constitute the main source for IL-4 in the nasal mucosa. Therefore, allergic patients have an increased synthesis of IL-4 when provoked with the allergen, and during natural pollen season this synthesis can be downregulated by corticosteroids. Furthermore, this study exemplifies the versatility of molecular biology in surgical pathology and that even low-copy-number cytokine mRNA can be examined in routinely snap-frozen surgical specimens.