The efficacy and safety of the nasal administration (twice in each nostril, four times a day) of levocabastine (0.5 mg/ml) were compared with those of sodium cromoglycate (20 mg/ml) and placebo in a 2-week, parallel, double-blind trial in patients with seasonal allergic rhinitis. At the end of treatment, 89% of patients in the levocabastine-treated group rated their treatment as globally good or excellent as compared with 32% (p = 0.003) of sodium cromoglycate-treated and 35% (p = 0.002) of placebo-treated patients. According to the investigators' ratings, the severest nasal symptom and ocular complaints responded better to levocabastine than to cromoglycate (p = 0.05 and p = 0.03) or placebo treatment (p = 0.001). Visual analogue scale ratings in patients' diaries indicated that at the end of therapy, nasal symptoms were less severe in the levocabastine-treated group than in the sodium cromoglycate-treated (p = 0.03) or placebo-treated group (p = 0.001). Total symptom severity as a percentage of the theoretical maximum symptom severity during the treatment period was lower for levocabastine than for sodium cromoglycate (p = 0.06) or placebo (p = 0.004) for the severest nasal symptom (35% versus 47% and 76%), sneezing (the most frequent symptom) (27% versus 42% and 67%), and itchy nose (18% versus 37% and 67%). The percentage of days at which nasal symptoms were entirely absent was markedly higher in the levocabastine-treated (33%) than in the sodium cromoglycate-treated (9%; p = 0.006) or placebo-treated (3%; p = 0.001) group. Adverse effects in the levocabastine-treated and sodium cromoglycate-treated group, and, in particular, daytime fatigue, did not occur more frequently than with placebo treatment. It is concluded that levocabastine is an efficacious and well-tolerated drug in the treatment of seasonal allergic rhinitis.