Optical coherence tomography (OCT) is a new noninvasive high-resolution method that measures the retinal nerve fiber layer (RNFL) thickness. An overview of the use in optic neuritis (ON) and multiple sclerosis (MS) is presented. Literature survey of PubMed was carried out. RNFL thickness in eyes of healthy control subjects was 102.9-111.11 microm, in eyes affected by ON 59.79-85 microm, and in fellow eyes 82.73-99.8 microm. All studies found a significant reduction in RNFL in eyes affected by ON compared with fellow eyes and eyes of healthy controls. Two out of three studies found a significant reduction in RNFL in fellow eyes compared with control eyes. RNFL thickness correlated with visual acuity, visual field, low-contrast letter acuity, contrast sensitivity, and color vision. Correlations were also found with the optic nerve area evaluated by magnetic resonance imaging, neurologic impairment score, and increasing disease duration. One of two studies found a significant correlation with amplitudes of visual evoked potentials, neither correlated with latencies. OCT is a promising new tool for evaluating atrophy in patients with ON and MS.