A 4 week, multicentre, double-blind, double dummy, placebo controlled group comparative study was carried out during the birch pollen season to compare the efficacy and tolerability of 2% nedocromil sodium eye drops (twice daily) and 2% sodium cromoglycate eye drops (four times daily). Participants with a history of seasonal allergic conjunctivitis (SAC) were randomized to receive nedocromil sodium (60), sodium cromoglycate (61) or placebo (64). Clinical assessment of SAC showed improvement with both active treatments compared to placebo but symptomatology was low and only changes in photophobia and grittiness reached significance (P < 0.05). Patient diaries showed significant control of itching by both active treatments, compared to placebo, with no differences between the active preparations. Patients' opinions indicated a marked placebo effect: 73% of this group reported full or moderate control of symptoms, compared with 75% in sodium cromoglycate and 80% in the nedocromil sodium group. Unusual symptoms were most common (27 patients) with nedocromil sodium eye drops: P < 0.05 vs. placebo (15 patients). There were no serious adverse events. Nedocromil sodium eye drops (b.d.) and sodium cromoglycate eye drops (q.i.d.) were both considered clinically more effective than placebo in controlling symptoms of SAC due to birch pollen.