The antihistamines astemizole and cetirizine were compared for the treatment of grass pollen hayfever in 158 patients who received either astemizole (10 mg od) or cetirizine (10 mg od) for a four week period in a randomised double-blind, double-dummy, parallel group, comparative assessment. Patients visited their general practitioner (GP) on three occasions and at each visit the GP made an assessment of the severity of individual hayfever symptoms. Patients recorded daily throughout the study the severity of their hayfever symptoms and their level of daytime sedation on visual analogue scales in a diary card. The primary measure of efficacy was the overall symptom visual analogue scores recorded daily by the patients. The results demonstrate that these two antihistamines are equally effective treatments for hayfever, in terms of magnitude of symptom relief, speed of onset of symptom improvement and lack of sedative effect.