BACKGROUND: Cetirizine is a highly sensitive H1 antihistamine with particular antiallergic properties, which has been shown to be effective in the treatment of allergic rhinitis, urticaria, and hay-fever-associated asthma. METHODS: To assess the effect of cetirizine on the late allergic reaction to a specific bronchial provocation test (BPT) with allergen, we selected 25 patients with allergic asthma as determined by history, skin tests, and specific IgE levels. They were challenged with increasing doses of a cat or mite extract until a 20% drop in forced expiratory volume in 1 second (FEV1) was recorded. Sixteen patients (11 men and 5 women with a mean age of 22 years; range, 18 to 48 years) exhibited a dual response (early and late allergic reactions). These 16 patients underwent a second BPT 2 weeks later, and each again showed a dual response. They were then randomized to receive either a placebo (8 patients) or cetirizine, 15 mg twice daily (8 patients) in a double-blind fashion. After 7 days of treatment, they underwent a third BPT with the same allergen dose as given in the second BPT. RESULTS: The intensity and duration of early allergic reaction were not affected by cetirizine, whereas all parameters of late allergic reaction were statistically significantly improved in the cetirizine group when compared with those of the placebo group (maximum FEV1 decrease, p = 0.046; FEV1 [area above the curve], p = 0.027; maximum airway resistance increase, p = 0.021; airway resistance [area under the curve], p = 0.036). CONCLUSIONS: Cetirizine produced a significant protective effect against an allergen-induced late allergic reaction in a BPT. Cetirizine might therefore be effective in the treatment of asthma.