Twenty five patients with optic neuritis (ON) of unknown etiology were treated with a high dosage of intravenous vitamin C. We measured blood levels of vitamin A, B1, B2, B6, B12, C, E, folate and zinc. All levels were compared with the normal values of our laboratory. The blood level of vitamin C (p < 0.001) was significantly less than the mean value of the normal. The blood levels of vitamin E, B6 (p < 0.01) and zinc (p < 0.001) also significantly decreased. Intravenous administration of vitamin C was given in those patients with decreased blood level of vitamin C. In order to compare the effect on vision by this treatment, the amplitude of recovery of vision, the time needed to attain the maximum vision, and the speed of visual recovery were analyzed. The results were compared with groups receiving other treatments. That is, Group A received intravenous administration of high dosage of vitamin C, Group B, intravenous pulse administration of corticosterone, Group C, oral administration of corticosterone, and Group D, oral administration of vitamin B12. Vision was significantly improved in all groups. There was no significant difference in improvement of visual acuity. Intravenous administration of vitamin C can be evaluated as the method of choice for the treatment of patients with ON. A possible mode of action by vitamin C on free radicals is discussed.