With a combination of red test stimulus and high background illumination, the test light illumination necessary to elicit a criterion amplitude or latency of the VER is compared for an eye suffering from optic neuritis and for the other eye. For various stages of optic neuritis (including slight changes of visual acuity), the increase of the VER threshold of the affected eye is more sensitive to optic nerve dysfunction than is the subjective sensory threshold. The clinical value of using the VER to detect minor changes of visual function produced by optic neuritis is further confirmed.