Background: Opticin is a recently discovered glycoprotein present predominantly in the vitreous humour. It is synthesised and secreted by the ciliary body epithelium (CBE) from the initiation of CBE development in the embryo, and production continues throughout life. Aim: To determine whether a variety of ciliary body tumours synthesise opticin to characterise further its role in ciliary body health and disease. Methods: Immunohistochemistry was used to determine the distribution of opticin in normal human CBE, and in hyperplastic and neoplastic CBE lesions. Results: Opticin was immunolocalised to lhe basal cell surface and basement membrane material of the non-pigmented CBE in nine donor eyes as well as four hyperplastic lesions of the CBE (Fuchs's adenoma). By contrast, none of eight neoplash'c lesions (two adenoma and six adenocarcinoma) of CBE stained for opticin. Conclusion: The present series supports the theory that opticin is produced by the non-pigmented CBE throughout adult life. Loss of opticin expression by this tissue is associated with and could contribute towards neoplastic transformation.