This is a clinicopathologic study of 12 human eyes that previously had been subjected to cyclocryotherapy. In ten cases detailed information was available regarding technical factors, such as temperature of the cryoprobe at the time of surgery and the like. The interval from treatment to enucleation ranged from 12 days to 4 1/2 years. On pathologic examination permanent, often massive, destruction and scarring of both layers of the ciliary body's epithelium was found in all 12 patients. The pigment epithelium was more susceptible to cryosurgical damage than was the nonpigment epithelium. Scarring of the ciliary body stroma and muscle occurred in 11 patients, scleral damage was present in 9, and a fibrous membrane lined the inner surface of the ciliary body in 5. This study indicates that cyclocryotherapy is an effective way of inducing permanent destruction of the human ciliary epithelium. Factors other than regrowth of epithelial cells account for failure to control the intraocular pressure in those patients in whom cyclocryotherapy proves ineffective.