BACKGROUND Antihistamine-decongestant combination products generally provide more benefit than individual components for adequately treating patients who have seasonal allergic rhinitis (SAR) with moderate-to-severe nasal congestion. OBJECTIVE To compare the effectiveness and safety of a 24-hour, extended-release formulation of desloratadine and pseudoephedrine with the individual components in patients who have SAR with moderate-to-severe nasal congestion. METHODS Patients with SAR and significant nasal congestion were enrolled in a multicenter, randomized, double-blind, double-dummy study. Patients were randomly assigned for 2 weeks to once-daily treatment with desloratadine-pseudoephedrine, 5/240-mg tablets; desloratadine, 5 mg; or pseudoephedrine, 240 mg. Primary efficacy variables for the antihistamine and decongestant components of desloratadine-pseudoephedrine were morning and evening reflective total symptom score (TSS), excluding nasal congestion, and morning and evening reflective nasal congestion score during the 2-week treatment period, respectively. Secondary variables included morning instantaneous (end-of-interval) TSS (excluding congestion), nasal congestion score, reflective morning and evening individual symptom scores, overall condition of SAR, and therapeutic response. RESULTS A total of 1121 patients were enrolled in the study, and 1047 patients completed the 2-week study. Desloratadine-pseudoephedrine was significantly more effective than desloratadine or pseudoephedrine monotherapy in reducing morning and evening reflective TSS (excluding nasal congestion) during the entire treatment period. Desloratadine-pseudoephedrine also was significantly more effective in reducing the morning and evening reflective nasal congestion score compared with desloratadine or pseudoephedrine monotherapy. Significant differences were seen between the desloratadine-pseudoephedrine and monotherapy groups for changes in morning instantaneous TSS (excluding nasal congestion) and nasal congestion scores. No unusual or unexpected adverse events were reported. CONCLUSIONS Desloratadine-pseudoephedrine, 5/240-mg once-daily tablets, provided additional benefit over desloratadine, 5 mg, or sustained-release pseudoephedrine, 240 mg, monotherapy in the treatment of patients with SAR and moderate-to-severe nasal congestion.