Patients with allergic conjunctivitis may experience several debilitating symptoms, particularly ocular itching. The objective of this study was to evaluate the efficacy and safety of pemirolast potassium 0.1% ophthalmic solution (Alamast trade mark ), a novel mast-cell stabilizer, for preventing ocular manifestations of seasonal allergic conjunctivitis. A pooled analysis was performed of data derived from 2 prospective, randomized, double-masked, placebo-controlled, multicenter phase III clinical trials of pemirolast potassium 0.1% in patients with a history of allergic conjunctivitis. Patients having a positive bilateral response to conjunctival allergen challenge (CAC) with ragweed antigen (N = 274) were randomized to receive pemirolast potassium 0.1% or placebo QID, beginning approximately 1-2 weeks before the onset of ragweed season and continuing until after the first killing frost (12-17 weeks duration). Patients recorded their daily evaluations of ocular itching in a diary. After the allergy season, patients underwent a second CAC. Evaluable patients (n = 265) recorded a total of 21,491 patient-days of ocular itching data during allergy season. In every 7-day or 14-day period, patients treated with pemirolast potassium 0.1% reported more days without any ocular itching compared with patients receiving placebo. Differences favoring pemirolast potassium 0.1% were statistically significant in 63% (10/16) of all 7-day periods (p < or = 0.046) and 88% (7/8) of all 14-day periods (p < or = 0.016). After the allergy season, pemirolast potassium 0.1% was significantly superior to placebo in relieving CAC-induced ocular itching, with relief occurring as early as 3 minutes after allergen challenge (p < or = 0.034). Pemirolast potassium 0.1% was well tolerated and had a safety profile similar to that of placebo. In conclusion, pemirolast potassium 0.1% is effective and safe in preventing ocular itching in patients with allergic conjunctivitis during allergy season.