The efficacy, tolerability, and adverse-effect profile of the recently developed, topical antihistamine levocabastine were compared with those of sodium cromoglycate in a 4-week double-blind, placebo-controlled trial in 95 patients with seasonal allergic conjunctivitis. At the end of the trial, global therapeutic efficacy was considered to be excellent or good in 87% of levocabastine-treated patients, as compared with 68% of sodium cromoglycate-treated patients (P= 0.006) and 63% of those who received placebo (P= 0.05). After 4 weeks of treatment, levocabastine patients had experienced significantly greater relief from their most severe ocular symptom than patients in the sodium cromoglycate group (P < 0.05). Furthermore, 37% of levocabastine-treated patients were virtually symptom-free for at least 75% of the treatment period, as compared with only 6% of patients in the sodium cromoglycate group (P < 0.01) and 4% of those who received placebo (P < 0.01). Moreover, this trend was maintained on days when the pollen count was high, when the corresponding figures were 33%, 6% (P= 0.02), and 4% (P= 0.02), respectively. Levocabastine was well tolerated. Indeed, the incidence of adverse reactions was no greater in the levocabastine group than in the placebo group. Topical levocabastine is an effective and well-tolerated drug for the prophylaxis and treatment of seasonal allergic conjunctivitis.