Thermal damage to the human sclera in relation to temperature and duration of exposure was studied in order to determine the heat tolerance of the sclera with respect to transscleral thermotherapy of choroidal melanoma. Samples of human sclera were submerged in saline for 10 sec to 10 min at temperatures of 37-100 degrees C. The effects of heat on the shape, weight and size of the samples were studied. Thermal damage of scleral collagen was examined by polarized light microscopy (LM) and electron microscopy (EM). The sclera was embedded in epoxy resin and stained with toluidine blue for LM and with uranyl acetate and lead citrate for EM. Thermal damage of scleral collagen on polarized LM was graded on a five point scale. Scleral damage was visible on macroscopic examination and on LM and EM in sclera heated at 65 degrees C for 20 sec or longer, at 70 degrees C for 10 sec or longer, and at higher temperatures. A sigmoidal function was used to define the relation between temperature and changes in diameter, thickness, and weight of scleral samples. Using fitted functions, the threshold temperature for thermal damage was estimated to be 59-61 degrees C when samples were heated for 10 min, 62-63 degrees C when heated for 1 min, and 66-67 degrees C when heated for 10 sec; the threshold exposure time at 60 degrees C was estimated to be 7-12 min. These results indicate a temperature of 60 degrees C for 1 min is well tolerated by human donor sclera; information of in vivo studies is required to validate whether this setting can be used in transscleral thermotherapy (TSTT) for choroidal melanoma.