BACKGROUND Cetirizine hydrochloride has proved effective in reducing allergic symptoms and can inhibit the infiltration of eosinophils in allergic late-phase responses in the skin. Because eosinophils are likely to play an important role in allergic late-phase reactions, we studied the effect of cetirizine on early and late asthmatic reactions and on levels of eosinophil cationic protein (ECP) in the blood after allergen challenge. METHODS The effect of 15 mg cetirizine given twice daily was studied in 16 patients allergic to house dust mites in a double-blind, placebo-controlled study. Patients were treated for 3 weeks. Before and after treatment, bronchial challenges with house-dust mites were performed. Blood ECP levels were measured 6 hours after challenge. Methacholine provocation was performed 72 hours before and 24 hours after each challenge. RESULTS Early and late asthmatic response--measured as mean maximal fall in forced expiratory volume in 1 second (FEV1) and the provocative dose of allergen that causes a 15% fall in FEV1 (PD15 allergen)--were not significantly reduced after treatment with cetirizine, compared with placebo. There was also no significant effect on the concentration of methacholine, which causes a 20% fall in FEV1 (PC20 methacholine). The increase in the level of ECP in the blood after allergen challenge was reduced after cetirizine treatment, compared with placebo, but this difference was not statistically significant. CONCLUSIONS Eighteen days of treatment with cetirizine did not significantly reduce the intensity of the early and late asthmatic responses.