Two per cent and 4% solutions of sodium cromoglycate eye drops were compared in patients with bilateral vernal keratoconjunctivitis. The trial was a double—blind within-patient group comparison and patients instilled 2% drops into one eye and 4% drops into the other eye four times daily for six weeks. Patients recorded their symptom severity each day on a diary card and clinical examinations were made at 0, 2, 4 and 6 weeks. Both 2% and 4% solutions of sodium cromoglycate were effective in treating the signs and symptoms of vernal keratoconjunctivitis. No difference in efficacy between the two solutions could be detected on the analysis of diary card scores or clinical assessments. The opinions of efficacy and treatment preferences recorded by patients and clinician at the end of the trial were very similar for each solution. Side effects were generally minor, usually relating to stinging following application of the eye drops.