Two hundred and seven patients with active symptoms of allergic conjunctivitis were studied in a four-week multicentre double blind, double dummy, clinical trial comparing efficacy and safety of nedocromil sodium eye drops with oral astemizole, placebo, and the combined active treatments. Clinical signs were evaluated by clinicians after 1, 2 and 4 weeks, while symptoms were recorded by patients on daily diary cards. Overall opinions were recorded at the end of treatment. Nedocromil sodium eye drops and astemizole, alone or combined, significantly reduced ocular symptoms when compared to placebo (diary card total symptom score and patients' opinion). In the clinicians' opinion nedocromil sodium alone or combined (but not astemizole alone) significantly decreased ocular symptoms when compared to placebo. All treatments were safe and well tolerated, drowsiness being the most frequent side effect observed in patients treated with astemizole. These results indicate the effectiveness of nedocromil sodium eye drops in the treatment of seasonal allergic conjunctivitis.