The efficacy and tolerance of topical administration (one drop in each eye q.i.d.) of levocabastine (0-5 mg/ml) was compared with that of sodium cromoglycate (20 mg/ml) and placebo in a 4-week double-blind trial in patients with seasonal allergic conjunctivitis. The investigator rated the treatment as globally good or excellent in significantly more patients treated with levocabastine (89%) than with cromoglycate (67%. P=0·03) or placebo (48%, P= 0·007). The patients felt that the treatment was more efficacious in 95% (levocabastine), 35% (cromoglycate) and 36% (placebo) of the cases in which they had taken previous antiallergic medication. Total symptom severity according to the patients' diary data was consistently lower with levocabastine than with cromoglycate or placebo for all ocular symptoms. The difference was mainly apparent at the beginning of treatment. The percentage of symptom-free days was higher in the levocabastine group (53%) than in the cromoglycate (31%, P= 0·02) and the placebo group (34%, P= 0·08). Particularly at high-pollen days, levocabastine was superior to cromoglycate in eliminating moderate or severe symptoms. Adverse effects did not occur more frequently with levocabastine or cromoglycate than with placebo. It is concluded that levocabastine is an efficacious, fast-acting and well-tolerated drug in the management of seasonal allergic conjunctivitis.