A double-blind study of terfenadine and placebo in 110 patients suffering from hay fever (confirmed by skin tests) was conducted. A novel technique was applied using an escape envelope containing a reference drug which could be taken under controlled conditions if, after 48 hours, the patient experienced no relief. Significantly more patients on placebo opened the envelope than patients taking the active drug. Terfenadine was demonstrated to be an effective drug in hay fever and produced no more drowsiness than placebo.