The lamina cribrosa is a sieve-like perforation in the posterior part of the sclera, that allows passage of the retinal ganglion cell axons and central retinal vessels and preserves a pressure gradient between the intraocular and extraocular space. It has been termed the primary site of glaucomatous damage to the optic nerve. Using electron microscopy, the authors morphometrically evaluated the inner surface of the lamina cribosa in 40 normal human donor eyes. There were 14 men and 21 women with a mean age of 52 +/- 22 yr (10-82 yr). Mean single pore area (0.004 +/- 0.001 mm2) and summed pore area were significantly (P less than 0.05) larger and the ratio of summed pore area to lamina area was higher in the inferior and superior regions than in the temporal and nasal regions. The ratio decreased with increasing lamina cribrosa size. Count, size, form, and density of the pores were statistically independent of age, sex, side, and lamina cribrosa form. Pore count and summed pore area (mean: 0.92 +/- 0.22 mm2) increased significantly with enlarging lamina cribrosa size. The area of the lamina cribrosa openings for passage of the central retinal vessels was independent of the lamina cribrosa size. The high ratio of summed pore area to lamina area and the large single pore area may be pathogenetically important for the increased glaucoma susceptibility in the inferior and superior disc regions. The lack of a correlation between lamina cribrosa size and the area of the lamina cribrosa openings for the retinal vessels may explain why central retinal vessel occlusions occur independently of optic disc size.