OBJECTIVE This study explored the safety and efficacy of cetirizine for treatment of allergic rhinitis and asthma. METHODS Daily treatment for 6 weeks with cetirizine 10 mg (93 patients) was compared with placebo treatment (93 patients) in a randomized, double-blind parallel study of patients with allergic rhinitis and asthma. This multicenter study was started just before onset of the fall pollen season. Rhinitis and asthma symptoms were assessed twice daily; spirometry was performed weekly. RESULTS Placebo-treated patients experienced a worsening of rhinitis symptoms from baseline throughout the study, whereas cetirizine-treated patients had a significant improvement in rhinitis symptoms at week 1, which was maintained after onset of the pollen season. Asthma symptoms in the cetirizine group improved from baseline at week 1; symptoms were significantly better than in the placebo group for 5 of 6 weeks of the study. Pulmonary function did not worsen in patients taking cetirizine or placebo; there were no differences between treatments as determined by spirometry. Albuterol use was less frequent in the cetirizine-treated patients for every week of the study, but differences did not reach significance. Pseudoephedrine use was similar in both groups. More cetirizine-treated patients (90%) completed the trial than did placebo-treated patients (74%). Both treatments were well tolerated. CONCLUSION Cetirizine 10 mg daily is safe and effective in relieving both upper and lower respiratory tract symptoms in patients with seasonal allergic rhinitis and concomitant asthma.