IN THE course of an investigation into the treatment of intraocular (Io) tumors by cryosurgery, we became aware that the penetration time and the appearance of ice on the surface of a tumor was distinct, and differed from that which occurred in the normal retina. We compared the appearance of ice on the surface of other lesions that might be confused with tumor and concluded that, with some experience, characteristic patterns can be recognized which are consistent and aid in the differential diagnosis of Io lesions. It is not our intention to review the results of the attempt to treat tumors, except to mention aspects of the investigation which are relevant to the present subject. The incentive for treating Io tumors came about with the realization that the cryosurgical application could penetrate sclera, and selectively destroy retinal and choroidal elements, without damaging the integrity of the scleral wall. Thus, it