AIMS/BACKGROUND--Eosinophil cationic protein (ECP) is a quantifiable product secreted by activated eosinophils. The aim of this study was to assess the degree of eosinophil activity in different clinical stages of various forms of allergic conjunctivitis. METHODS--Tears were collected in glass capillary tubes from 14 subjects with seasonal allergic conjunctivitis (SAC), 23 subjects with vernal keratoconjunctivitis (VKC), 16 subjects with atopic keratoconjunctivitis (AKC), 10 subjects with giant papillary conjunctivitis (GPC), and 16 healthy control subjects. The samples were analysed in duplicate with a radio-immunoassay for ECP. RESULTS--Statistically significant differences were evident between healthy controls and allergic subjects (p < 0.001). Subjects with AKC and VKC had significantly higher tear ECP values than subjects with GPC and SAC. In addition, there was a significant correlation between ECP values and disease severity in all disorders. CONCLUSION--The data suggest a particular pathogenic role of the eosinophil in VKC and AKC, and a less pronounced but still important eosinophil involvement in the disease processes of GPC and SAC.