Using a specular microscope and a pachymeter, we monitored endothelial cell patterns and central corneal thickness in 16 cases of clear graft with different postoperative courses for over 2 years after successful penetrating keratoplasty. With the use of a computerized digitizer, the graft endothelium was analyzed to determine the mean cell size, standard deviation of cell sizes and the coefficient of variation in cell size. In all cases, a progressive increase in the mean and standard deviation of cell sizes was noted throughout the period of observation. The coefficient of variation, however, stayed fairly constant in each case. This indicates that the increase in the standard deviation of cell sizes is dependent on the increase in the mean cell size. Five cases with good postoperative course showed gradual alterations in cellular morphology. On the other hand, the endothelium of 5 cases with surgical intervention after keratoplasty changed its morphology greatly until the corneal thickness approximated normal, and then the alterations continued to take place gradually. In 6 cases, a remarkable and rapid increase in the mean and standard deviation of cell sizes occurred during the rejection process. These parameters continued to increase gradually after the graft became transparent as the result of intensive corticosteroid therapy. These results suggest that the graft endothelium is still in a transitional state during healing even 2 years after keratoplasty, and that the morphologic parameters determined during the early postoperative period reflect the actual cell damage owing to surgical procedures to some extent but not exactly.