Astemizole, a new long-acting antihistamine, (H1 receptor antagonist), 10 mg taken once daily was found to be an effective prophylactic treatment for the symptoms of allergic rhinitis in 75% (twelve from sixteen) of subjects in a double-blind placebo-controlled study. The success rate with placebo was < 20% (three from sixteen). There were two treatment failures in highly atopic individuals taking the active drug. The classic side effects of antihistamines, most notably sedation, were minimal and reflected in the control group. Two subjects reported weight gain: one of these noted an increase in appetite during the 6-week trial period. Six of seven subjects who continued to take the drug for a longer period (up to 16 weeks) experienced marked weight gain: five of them reported an increase in appetile. Reactions to allergen skin tests were significantly reduced in patients taking astemizole.