The protective effect of terfenadine on inflammatory processes following the early phase of conjunctival provocation tests by specific allergen was assessed in 24 patients suffering from seasonal allergic rhinoconjunctivitis, in a single-blind, randomized, placebo-controlled, parallel-group study. Patients were randomly assigned to four treatment groups, each being given terfenadine 60, 120, 180 mg daily, or placebo, respectively, for 7 days, out of the pollen season. Clinical severity (burning, itching, lacrimation and hyperemia), and number of inflammatory cells obtained by conjunctival scraping (neutrophils, eosinophils, lymphocytes, and monocytes) were evaluated before and after treatment. Pre-treatment with terfenadine resulted in a significantly higher allergen threshold dose than placebo (p less than 0.01), regardless of drug dose. Patients given terfenadine experienced a significant reduction (p less than 0.03) in conjunctival symptom severity, as compared with placebo, following conjunctival challenge. Accordingly, the number of inflammatory cells in terfenadine-treated patients was significantly lower (p less than 0.01) than in the placebo-treated after the conjunctival provocation test. The results of this study suggest that terfenadine has a significant protective effect on the early-phase cellular and clinical events of conjunctival reaction induced by allergen challenge in atopic patients.