In a double-blind, multicenter study, we compared the effects of SCH 434 (Claritin-D; Schering Corp., Kenilworth, N.J.), a new sustained-release, combination antihistamine/decongestant medication, with the effects of its individual components and placebo in 435 patients with seasonal allergic rhinitis. SCH 434 contains 5 mg of loratadine, a nonsedating antihistamine, and 120 mg of pseudoephedrine as the decongestant component. Administered twice daily in this study, SCH 434 effected a 50% decrease in total symptom scores at day 4 and was significantly (p less than or equal to 0.03) more effective than the components alone or the placebo. Loratadine or pseudoephedrine alone, with 43% and 33% decline in symptom scores, respectively, also was more effective than placebo (p less than 0.05). As expected, pseudoephedrine alone was more effective than loratadine (p less than 0.01) in relieving nasal stuffiness; SCH 434 was more effective (p less than or equal to 0.01) than placebo and loratadine in relieving nasal stuffiness. All treatments were safe and well tolerated, although insomnia and dry mouth were noted in a significant number of patients who received either SCH 434 or pseudoephedrine. No serious side effects were noted. The incidence of sedation did not differ significantly among the four treatment groups. We conclude that SCH 434 is a safe and effective treatment for symptoms of seasonal allergic rhinitis. The combination drug (SCH 434) was better than its components for some, but not all, symptoms.