Background The purpose of this study was to evaluate the efficacy and duration of action of once-daily dosing with alcaftadine 0.25% ophthalmic solution and olopatadine 0.2% ophthalmic solution as compared with placebo in the prevention of ocular itching, and to directly compare the efficacy of alcaftadine 0.25% with olopatadine 0.2% in the prevention of ocular itching associated with allergic conjunctivitis using the conjunctival allergen challenge model. Methods Subjects with allergic conjunctivitis (n = 127) were enrolled in a multicenter, double-masked, randomized, active-controlled and placebo-controlled clinical trial. Using the conjunctival allergen challenge model, this study was conducted over the course of approximately 5 weeks. Subjects were randomized into one of three treatment arms: alcaftadine 0.25% ophthalmic solution, olopatadine 0.2% ophthalmic solution, or placebo. Study medications were administered twice over the course of the trial. The primary efficacy measure for the study was ocular itching evaluated by the subject at 3, 5, and 7 minutes post challenge. Secondary endpoints, measured at 7, 15, and 20 minutes post challenge, included conjunctival, ciliary, and episcleral redness, lid swelling, chemosis, and tearing. Duration of action was measured at 16 and 24 hours post-instillation of the study medication at visits 3 and 4, respectively. Results For the primary measure of ocular itching, both actives, alcaftadine 0.25% and olopatadine 0.2%, were statistically superior to placebo at all three measured time points for both the 16-hour and 24-hour measures (P < 0.0001). Eyes treated with alcaftadine 0.25% had numerically lower mean ocular itching scores than eyes treated with olopatadine 0.2% at every time point, and this difference was statistically significant at the 3-minute time point 16 hours post instillation (P = 0.026). Eyes treated with alcaftadine 0.25% and with olopatadine 0.2% displayed significantly less lid swelling relative to placebo at every time point for the 16-hour and 24-hour post-instillation visits (P < 0.005). Alcaftadine 0.25% was the only active treatment that provided statistically significant relief of chemosis at every time point of the 24-hour post-instillation visit. Conclusion Both the alcaftadine 0.25% and olopatadine 0.2% ophthalmic solutions provided highly effective relief of ocular itching at both 16 and 24 hours post-instillation. Treatment differences between the actives were most pronounced at the earliest time point (3 minutes post-challenge) following conjunctival allergen challenge (16 hours), when alcaftadine 0.25% ophthalmic solution was statistically superior to olopatadine 0.2% ophthalmic solution. Alcaftadine 0.25% was the only treatment to provide significant relief from chemosis at both 16 and 24 hours post-instillation. Both active treatments and placebo were generally safe and well tolerated.