The corneal endothelial cell mosaic is composed of polygonal cells, yet it is unknown why the cells have different sizes and shapes. To explore possible physical reasons for this, sets of scanning electron micrographs from young albino rabbits and specular micrographs from moderately aged Caucasian adult humans were subjected to morphometry. The apical surface areas of 80-120 contiguous cells from each central corneal endothelium were measured by planimetry and the number of cell sides recorded for each cell. Comparisons between the average cell areas and the number of cell sides yielded a linear relationship for cells with four to eight sides, with the cell areas also showing a linear relationship with respect to the six-sided cells. These results lend support to the idea that geometric determinants of cell area play an important role in the determination of cell size and their packing in the human corneal endothelium. These types of interrelationships could also apply to other endothelia.