Four unusual patients had bilateral anterior ischemic optic neuropathy (AION). In all four cases, AION developed in the first eye with the classic presentation. The fellow eye had symptomless optic disc edema (ODE) with no subjective or objective visual loss initially, but the classic AION clinical picture developed later on. The findings indicate that symptomless ODE may precede the visual loss in AION and could constitute the earliest sign of this disease. Since ODE in AION is due to axoplasmic flow stasis that, by itself, does not produce visual loss (this is produced by disruption of visual impulse transmission), this would suggest that mild optic nerve head ischemia interferes with axoplasmic flow without disrupting the visual impulse; however, more severe ischemia would disrupt both. Possible effects of various grades of acute optic nerve head ischemia are discussed.