Clinical data and fundus fluorescein angiograms were analyzed from 35 patients with acute (onset less than four weeks) anterior ischemic optic neuropathy. Nineteen of the 35 patients (54%) had nonarteritic disease, and 16 patients (46%) had giant cell arteritis confirmed by biopsy. Patients with arteritis had higher erythrocyte sedimentation rates, larger cup/disk ratios, and delayed fluorescein dye appearance and choroidal filling times. Three additional patients with cranial arteritis confirmed by biopsy, but without visual loss, had angiographic characteristics similar to patients with arteritic ischemic neuropathy. We consider fluorescein angiography a valuable diagnostic adjunct in identifying patients with giant cell arteritis.