Although the clinical features of atopic keratoconjunctivitis have been well described, it is recognised that some adults with severe chronic allergic conjunctivitis do not develop a keratopathy despite having marked disease of their lid margins and conjunctiva. The aetiology of the keratopathy of chronic allergic eye disease is not known but may result from the release of toxic mediators from mast cells, eosinophils and neutrophils in the inflamed conjunctiva. To examine this theory we have counted these cells in tarsal and bulbar conjunctival biopsies from patients with severe allergic conjunctivitis and compared the numbers in samples from patients with or without keratopathy. Our results indicate that patients with chronic allergic conjunctivitis with keratopathy have higher cell numbers in their conjunctiva than patients with no keratopathy, especially those staining for eosinophil cationic protein in the tarsal epithelium. A statistically significant difference between disease groups could only be demonstrated, however, when the combined totals of mast cells, eosinophils and neutrophils were compared.