Olopatadine hydrochloride ophthalmic solution 0.2% (Pataday, Alcon) is a new formulation of olopatadine hydrochloride ophthalmic solution, the first topical ocular antiallergic agent indicated for once-daily dosing. The aim of this study was to evaluate the safety, efficacy, onset, and duration of action of olopatadine 0.2% in the treatment of allergic conjunctivitis. Using the conjunctival allergen challenge, this double-masked, randomized by eye, parallel-group study included four visits over a 5-week period. Subjects were screened for eligibility (visit 1) and their ocular allergic responses were confirmed at visit 2. The efficacy of olopatadine in reducing the signs and symptoms of allergic conjunctivitis was evaluated at onset of action (visit 4) and 16 hours (visit 3) after masked medication instillation. The primary efficacy parameter was ocular itching. Safety parameters were also evaluated. Ninety subjects were evaluated. Olopatadine 0.2% was significantly (p < 0.001) more effective than placebo in the treatment of ocular itching at all time points at both the onset of action and the 16-hour allergen challenges. Olopatadine 0.2% was significantly (p < 0.03) more effective than placebo in the reduction of conjunctival redness, chemosis, and eyelid swelling at all time points (with the exception of conjunctival redness, which was significantly reduced at five of six time points). There were no serious adverse events and no treatment-related adverse events. Once-daily dosing with olopatadine 0.2% reduced the signs and symptoms of allergic conjunctivitis with a rapid and prolonged duration of action. Safety analyses indicated that olopatadine 0.2% was safe and well tolerated in subjects with a history of allergic conjunctivitis.