To investigate the role of the eosinophil in vernal keratoconjunctivitis and contact lens-associated giant papillary conjunctivitis, we assessed the presence of eosinophil granule major basic protein in conjunctival tissues by immunofluorescence. Biopsy specimens of conjunctiva were taken from nine patients with vernal keratoconjunctivitis, seven patients with giant papillary conjunctivitis, and five control subjects. We performed a masked semiquantitative assessment of immunofluorescence on sections from each specimen. The vernal keratoconjunctivitis and giant papillary conjunctivitis groups had significantly (P less than .05) more major basic protein deposition than controls. No significant correlation between severity of disease and degree of major basic protein deposition was found. We found extracellular eosinophil granules in one of three vernal keratoconjunctivitis specimens examined by transmission electron microscopy. Thus, eosinophil degranulation commonly occurs in vernal keratoconjunctivitis and giant papillary conjunctivitis with release of eosinophil granule major basic protein and presumably other toxic granule proteins onto affected tissues. These cationic proteins are potent cytotoxins and are able to stimulate mast cell degranulation.