This randomized, double-masked, active-control, parallel-group trial compared the mast cell stabilizers pemirolast potassium 0.1% and nedocromil sodium 2% in the treatment of seasonal allergic conjunctivitis. Pemirolast is currently indicated for four-times-daily administration, nedocromil, for twice-daily dosing. Both ophthalmic solutions were instilled bilaterally twice a day for 8 weeks. The study involved four office visits and two telephone contacts. Participants evaluated their symptoms daily in take-home diaries (itching was the primary efficacy variable) and completed questionnaires to assess comfort. Of a total enrollment of 80, 78 patients completed the study. No significant differences were found between pemirolast and nedocromil on any signs or symptoms of allergic conjunctivitis (redness, chemosis, itching, eyelid swelling). At each visit, pemirolast was rated significantly more comfortable than nedocromil. A significantly higher percentage of the pemirolast group experienced no signs or symptoms at work or school (58% vs 28%; P = .005). The number of adverse events did not differ significantly between groups. Twice-daily administration of pemirolast potassium was as efficacious and safe as twice-daily nedocromil sodium in the 8-week treatment of ragweed allergic conjunctivitis and was superior to nedocromil in comfort. Increased comfort with pemirolast may increase patient satisfaction and compliance with therapy.