A total of 22 asymptomatic patients with a documented history of allergic rhinitis participated in this single-centre, double-blind, randomized, placebo-controlled, cross-over trial undertaken to assess the efficacy and tolerability of levocabastine nasal spray (0.5 mg/ml) in the prevention of allergen-induced nasal symptoms. Objective assessment of nasal symptoms revealed that the severity of sneezing was significantly lower following treatment with levocabastine (p < 0.001), with rhinorrhoea also tending to be less severe in the levocabastine-treated group (0.05 < p < 0.1). Rhinomanometry and acoustic rhinometry failed to reveal any significant intergroup differences, and there were no differences in nasal albumin concentrations between the two treatment groups. Patients' VAS ratings revealed significant differences in favour of levocabastine for sneezing (p < 0.001) and itching (p < 0.05), with the severity of rhinorrhoea also tending to be lower during treatment with this topical antihistamine (0.05 < p < 0.1). The mean total symptom score was also significantly lower in levocabastine-treated patients (p < 0.05). Levocabastine was well tolerated. Only two adverse events were reported: fatigue in one patient, and vesicular rash with facial oedema and urticaria in another. In conclusion, intranasal levocabastine provided effective protection from nasal allergen challenge and would appear to be a valuable therapeutic approach in patients with allergic rhinitis.