Fluorescein angiography of the optic disc was performed on normal subjects, patients with nonmyopic open-angle glaucoma dna myopic open-angle glaucoma, normal myopes, patients with optic atrophy due to chiasmal pituitary tumors, and patients with sectorial ischemic optic neuropathy due to vascular hypotension. The normal myopes, those with optic atrophy secondary to pituitary tumor, and the ischemic optic neuropathy group had optic discs similar in appearance to those with open-angle glaucoma. Absolute fluorescein filling defects occurred only in patients with open-angle glaucoma and sectorial ischemic optic neuropathy. The filling defects in open-angle glaucoma appear to be specific, indicating that the blood supply to the anterior portion of the optic nerve is involved, as in ischemic optic neuropathy. Fluorescein angiography of the optic disc may be useful to differentiate open-angle glaucoma from other entities that have similar optic discs.