BACKGROUND: Clinically apparent relief of nasal symptoms of allergic rhinitis is generally recognized to occur within 3 days to 1 week when intranasal corticosteroids are used. OBJECTIVE: A study was designed to evaluate the onset of action of triamcinolone acetonide (TA) in patients with ragweed-induced allergic rhinitis with an environmental exposure unit (EEU). METHODS: Eighty-five adults with ragweed-induced allergic rhinitis were primed with ragweed allergen in the EEU. Symptoms were recorded during a baseline exposure in the EEU, and subjects were randomized with a 5:1 ratio to receive either TA 400 μg (n = 71) or its propellant (n = 14). Subjects received study medication for 7 days under supervision in the morning and returned to the EEU in the evening for ragweed allergen challenge and symptom assessment. Clinically apparent onset of action was defined as a 25% decrease in symptom scores from baseline. RESULTS: A mean reduction in nasal congestion from baseline of greater than 25% (onset of action) was observed in the TA group, but not in the placebo group, by 10 hours. This was also observed for itching of the nose or palate and a combined measure of symptoms. In addition, the proportion of subjects with less nasal congestion after 1 day of treatment was greater in the TA group (41%) than in the placebo group (7%) (p < 0.05). CONCLUSION: The unexpected early relief of symptoms observed in the TA group and, to a lesser extent in the placebo group, has important clinical implications in the treatment of allergic rhinitis. (J ALLERGY CLIN IMMUNOL 1996;97:1050-7.)