A retrospective analysis of the clinical features of 81 patients with acute idiopathic optic neuritis and 58 patients with nonarteritic anterior ischemic optic neuropathy revealed a surprising overlap of manifestations. The rate of visual decline and the range of visual acuities were the same for both. Central scotomas and improvement in acuity were more common in optic neuritis, but occurred often enough in nonarteritic anterior ischemic optic neuropathy to limit their value as single diagnostic criteria. The similarities observed in this study suggest that it may be difficult to differentiate between optic neuritis and nonarteritic anterior ischemic optic neuropathy solely on nosologic grounds in some instances of acute, unilateral optic neuropathy.