The prophylactic effectiveness of the new, second-generation antihistamines cetirizine and terfenadine was evaluated in a double-blind, multicenter study of 487 patients with seasonal allergic rhinitis. The patients were randomly assigned to eight weeks of treatment with 10 mg of cetirizine given either in the morning or evening, or 60 mg of terfenadine twice daily. Success rates in the three treatment groups, as assessed by physicians, were 61%, 58%, and 56%, respectively. On a scale of 0 (no symptoms) to 100 (most severe symptoms), symptoms in the three regimens were rated by patients as 31, 34, and 41, respectively. A significantly greater alleviation of eye watering and irritation was noted in the morning cetirizine group than in the terfenadine group. No serious side effects were experienced. It is concluded that 10 mg of cetirizine once daily is as effective as 60 mg of terfenadine twice daily in the prophylactic treatment of seasonal allergic rhinitis.