A multicenter double-blind group comparative study was carried out in 126 patients, 64 of whom received 2% nedocromil sodium eye drops, and 62 placebo eyedrops twice each day for the treatment of seasonal allergic conjunctivitis to birch pollen. The patients were evaluated at 2 week intervals for clinical signs of conjunctivitis and kept daily diary records of eye symptoms (0-4 severity scales) and concomitant therapy. Diary trends favored active treatment, and reached significance for excessive lacrimation (P less than .05) during peak pollen challenge. Clinic assessments showed the same directional trend and final opinions of treatment efficacy were significantly in favor of nedocromil sodium (P = .003, patients; P = .006, clinicians). In addition, the placebo group used significantly more topical (P less than .05) and oral (P less than .01) concomitant antihistamine therapy. Nedocromil sodium and placebo treatments were equally acceptable and well tolerated. The results show that 2% nedocromil sodium used topically twice each day is an effective therapy for seasonal allergic conjunctivitis.