The efficacy and safety of twice-daily nedocromil sodium 2% ophthalmic solution and vehicle were compared in the treatment of ragweed seasonal allergic conjunctivitis. Two separate multicenter, randomized, double-masked, placebo-controlled studies were subjected to a combined analysis. Following a one-week baseline period during the beginning of the ragweed pollen season, 189 patients with seasonal allergic conjunctivitis received either nedocromil sodium or vehicle b.i.d. for eight weeks. Efficacy was evaluated by patient diary cards and clinical eye examinations. Safety was assessed by reports of adverse events. Compared with vehicle, nedocromil sodium produced significantly greater decreases in summary symptom score (p = 0.005), itch (p = 0.005), tearing (p = 0.004), overall eye condition (p = 0.001), and clinician-evaluated conjunctival edema (p = 0.018), and significantly better (p = 0.001), and patient (p = 0.001) opinions of treatment effectiveness at the peak pollen period. Additionally, the superiority of nedocromil sodium compared to vehicle approached statistical significance in redness reduction (p = 0.087) and clinician-evaluated conjunctival injection (p = 0.087). There were no serious treatment-related adverse events in either treatment group. In summary, nedocromil sodium 2% ophthalmic solution b.i.d. was found to be effective and to have a favorable safety profile in the treatment of seasonal allergic conjunctivitis.