We compared the efficacy and safety of cetirizine (5 mg), pseudoephedrine retard (120 mg), and the combination of cetirizine (5 mg) with pseudoephedrine retard (120 mg), each given twice daily for two weeks to subjects with pollen-associated allergic rhinitis. The study was multicentre and of randomized, double-blind, parallel-group design. Five rhinitis symptoms were rated according to severity on a scale of 0 - 3, daily by patients and at each clinic visit by investigators. A total of 687 patients, aged 9 - 66 years (mean: 32 years) was randomised to treatment (cetirizine: 231; pseudoephedrine: 226; combination: 230). On entry, the three groups were comparable in relevant respects. The primary outcome measure was based on the five symptoms assessed by the patients over the 2-week treatment period. The combination was more effective, providing at least 20% more "comfortable days" (symptoms absent or at most mild) than cetirizine or pseudoephedrine given alone (median values: 53.3%, 30.8%, and 33.3%, respectively; p < 0.001). For nasal obstruction, the combination (mean score: 1.19) was more effective than cetirizine (mean score: 1.43; p = 0.0005), but there was little difference between the combination and pseudoephedrine (mean score: 1.22; not significant). Sneezing, rhinorrhoea, nasal and ocular pruritus were better controlled by combination (mean 4-symptom score: 0.77) than by pseudoephedrine alone (mean 4-symptom score: 1.12; p < 0.001) and also better than by cetirizine alone (mean 4-symptom score: 0.93; p < 0.001). No unexpected adverse reactions were observed. A combination of cetirizine and pseudoephedrine retard is well tolerated and superior to each given alone for moderate to severe allergic seasonal rhinitis, especially when nasal obstruction is a predominant symptom.