We measured the horizontal diameter, vertical diameter, and area of the optic disk in fellow eyes of patients with unilateral nonarteritic ischemic optic neuropathy and in control eyes without ocular disease. The fellow eyes of patients with anterior ischemic optic neuropathy had a smaller horizontal than vertical disk diameter (P less than .05), a difference not present in controls. Both the horizontal disk diameter and the disk area were significantly smaller in fellow eyes of patients with anterior ischemic optic neuropathy than in controls (P less than .05). There was no statistical difference in the vertical disk diameters between the two groups. Our findings suggest that a small disk area along with a horizontal shortening of the scleral canal can lead to crowding of optic nerve fibers, predisposing to a circulatory compromise of the optic nerve head in non-arteritic anterior ischemic optic neuropathy.