We examined the histologic structure of the optic nerve head in 15 eyes of nine persons with a known glaucoma history. All had been seeing eyes, varying from normal visual acuity and visual field to advanced glaucoma damage. The site of damage to nerve fibers is the scleral lamina cribrosa, where there is local blockage of axonal transport. Early cup size increase prior to definite field loss results from loss of nerve fibers, not from damage to astrocytic glial cells of the nerve head. No selective damage to nerve head capillaries is seen in mildly damaged specimens. Scanning electron microscopic analysis suggests that the structure of the lamina cribrosa is an important determinant of the degree of susceptibility to damage by elevated intraocular pressure.