To test the hypothesis that nasal antiinflammatory treatment can modify both upper and lower airway responses to allergen exposure, 12 cat-allergic subjects underwent 1 h cat exposure challenges at baseline, with nasal occlusion, and after 1 wk of treatment with either intranasal triamcinolone acetonide or placebo in a double-blind crossover trial. Challenges were performed in a room containing two cats with airborne Fel d I levels ranging from 35 to 37,525 ng/m3. Overall, nasal symptoms were moderately reduced by treatment (p = 0.06), with the greatest reduction occurring in the first 15 and 30 min of the challenge (p < 0.01 and p < 0.05, respectively). Mean lower respiratory symptoms were also diminished by treatment (p = 0.02), although those effects were most evident during the last 15 min of the challenge. Maximum changes in FEV1 were slightly reduced by the nasal therapy (p = 0.07), reaching statistical significance only at the 30-min intervals (p < 0.05). There were no significant differences in nasal histamine or TAME esterase levels. When challenges were repeated with nasal occlusion, no significant differences were detected in chest symptoms or FEV1 changes. We conclude that treatment with an intranasal corticosteroid led to significant reductions in both upper and lower airway responses to intense cat exposure.