Terfenadine suspension, 30 mg b.i.d., was compared with placebo in a randomized, double-blind cross-over study in twenty-five children, 6–12 years of age, with grass pollen induced allergic rhinoconjunctivitis. The patients were treated during two 7-day periods separated by a 4-day wash-out period. Efficacy was assessed during a period without provoking pollen in the air. At the end of each treatment period, skin-prick tests were carried out in quadruplicate with three concentrations of grass pollen extracts (identical batches of Pharmalgen®) and histamine HCl, 1 and 10 mg/ml, as were conjunctival provocations with the same grass pollen. The mean size of weals caused by allergen and histamine was significantly smaller after terfenadine than after placebo; in fact, terfenadine increased the tolerance to the allergen by a factor of ten. Similarly, the tolerance to conjunctival provocation was significantly increased during terfenadine treatment as compared with placebo. There was no significant difference between the treatments in scores for alertness and salivation. Seasonal symptoms were mild when the children were allowed to use terfenadine in an open follow-up study. Thus, terfenadine reduced specific as well as non-specific reactivity in grass pollen allergic children and caused few side effects.