No study has yet been done to investigate the changes in endothelial cell size, perimeter, and density that may result from the warming of corneas in MK (McCarey-Kaufman) medium for specular microscopy. In the present investigation eye bank eyes were stored in MK medium at 4 degrees C and rewarmed daily for six days at 37 degrees C before specular photography of the endothelium was performed. These photographs were compared with wet mount preparations stained with trypan blue and alizarin red made from the same corneas and those stored without rewarming for six days. In addition all corneas were qualitatively analysed with the scanning electron microscope (SEM). The data from serial specular photography were insufficient to allow significant conclusions to be drawn about day to day changes in cell morphology. However, analysis of wet mount preparations revealed that cell density and perimeter varied significantly between those corneas rewarmed daily and those held in cold storage for six days. SEM studies showed an intact cell monolayer with cell loss along the folds of corneal endothelium. We therefore concluded that repeated rewarming at 37 degrees C of corneas stored in MK medium at 4 degrees has a deleterious effect on cell morphology and that folds induced by swelling of corneal tissue result in endothelial cell damage with some loss.