This study aimed to assess the relationship between the rate of nerve fiber loss in non-arteritic anterior ischemic optic neuropathy (NAION) and time delay before therapy. Total 24 patients received the same treatment within or after 2wk (early and late groups). There were significantly lower level of destruction of nerve fibers (P=0.0014) and significantly better visual field sensitivity (P=0.039) in early group. The results indicate that therapy should be started within 2wk. The degree of ischemic damage due to NAION correlates well with retinal nerve fiber layer thickness and the ischemia-induced decrease in visual field sensitivity.