The association between optic neuritis and multiple sclerosis is well established. Optic neuritis may be the first manifestation of multiple sclerosis, or it may occur later in its course. A strong case can be made for "isolated" optic neuritis being a forme fruste of multiple sclerosis, based on similarities between the two in such epidemiologic factors as gender, age, geographic distributions, cerebrospinal fluid changes, histocompatibility data, magnetic resonance imaging (MRI) changes, and family history. The magnitude of the risk of multiple sclerosis after optic neuritis is uncertain. Previous studies have reported very disparate results, with the risk being reported to be as low as 13 percent and as high as 88 percent. The importance of risk factors such as age, gender, and MRI changes in predicting which patients with optic neuritis are most likely to develop multiple sclerosis also is unclear.