Environmental challenge chambers (ECC) have been used to expose people to pollen allergens within a stable atmosphere and to examine the efficacy of treatment. Although pollinosis is one of the typical IgE-mediated type I allergic diseases, allergic inflammation is thought to contribute to the fundamental pathogenesis and prophylactic treatment may reduce exacerbations of pollinosis. The purpose of this study was to compare the efficacy of prophylactic treatment with nasal steroid (mometasone furoate nasal spray) or an antihistamine (fexofenadine) in the control of cedar pollinosis using the ECC. In a randomized, double-blind two-way crossover study, 48 patients received nasal steroid or antihistamine for 7 consecutive days (days 1-7). On day 8, patients were exposed to cedar pollen (8000 grains/m(3)) in the ECC for 3 hours. Nasal symptoms induced by pollen exposure were assessed. Total nasal symptom scores (TNSSs) during the exposure in the ECC were not significantly different between the antihistamine and the nasal steroid groups. Nasal symptoms induced by pollen exposure using the ECC persisted for up to 3 days. TNSSs after pollen exposure on days 8-11 were significantly lower in the nasal steroid group compared with the antihistamine group. Prophylactic treatment with nasal steroid is more effective than antihistamine against pollinosis, particularly in the late phase. Clinical trial registration JAPIC CTI 101182 (www.clinicaltrials.jp/user/ctiMain_e.jsp).