This single-masked, contralateral-eye, active-controlled allergen-challenge study compared ketotifen fumarate .025% and cromolyn sodium 4% ophthalmic solutions in the prevention of ocular itching, tearing, and redness induced by allergen challenge. After a confirmatory conjunctival provocation test (CPT), 56 patients randomly received masked study medication (placebo in one eye, cromolyn in the other eye) four times daily for 2 weeks. At visit 3, patients received one drop of ketotifen in the eye previously treated with placebo and cromolyn in the other eye. Ocular comfort was assessed 30 seconds postinstillation, and a CPT was conducted 15 minutes and 4 hours postinstillation to evaluate ocular itching, tearing, and redness. Forty-seven patients were analyzed for efficacy. At the 15-minute and 4-hour challenges, ketotifen was superior to cromolyn in preventing itching (P<.001) at all assessments and redness (ciliary, conjunctival, and episcleral) (P≤.001 ) at most assessments. Tearing scores were higher in cromolyn-treated eyes than in ketotifen-treated eyes. Patients reported greater comfort in the ketotifentreated than in the cromolyn-treated eye (P= .066). The most common adverse event was burning/stinging with cromolyn. A single dose of ketotifen was superior to a 2-week four-times-daily regimen of cromolyn in alleviating symptoms of allergic conjunctivitis in the conjunctival allergen-challenge model.