Quantitative specular endothelial microscopy was performed on 39 donor corneas before operation and at three and 12 months following penetrating keratoplasty. The central endothelial cell density of these corneas had decreased by 18.3% at three months after operation and by 33.6% at the end of one year. This decrease in cell density probably results both from cell death and from migration of cells toward the peripheral cornea. Postoperative cell counts were correlated closely with preoperative counts but not with donor age. Therefore, it is important to screen donor corneas before operation with the clinical specular microscope.