The efficacy of loratadine as prophylactic therapy for seasonal allergic rhinitis was evaluated in a randomized, double-blind, parallel group, placebo-controlled study. One hundred eighteen subjects received either loratadine, 10 mg once daily, or placebo for 6 weeks. Treatment was begun prior to the onset of grass pollen seasonal symptoms of allergic rhinitis. Total symptom-free days occurred more frequently in subjects receiving loratadine. More loratadine than placebo subjects (65% versus 49%) had no symptoms or mild rhinitis at the end of the study. In contrast, the differences between loratadine and placebo in symptom scores did not achieve significance. The incidence of sedation and anticholinergic effects were comparable between the groups. Prophylactic loratadine therapy was effective in suppressing symptoms of seasonal allergic rhinitis and providing patients with symptom-free days throughout the pollen season.