For lack of a more precise definition, optic neuritis is an acute, noninfectious inflammatory optic neuropathy that predominantly affects young people. By the end of the 19th century, the clinical profile of this disorder and its relationship to multiple sclerosis (MS) were well characterized.1- 3 In the subsequent 100 years, numerous reports identifying erroneous etiologies and seemingly successful treatments were published; however, we still lacked the complete clinical profile and an understanding of its natural history. The value of treating optic neuritis with corticosteroids, a treatment first available in the 1950s, remained controversial. Although several investigators had reported that corticosteroid treatment did not alter the final visual outcome, neurologists and ophthalmologists tended to treat optic neuritis patients with oral prednisone, especially if, as is often the case, the patients had severe loss of vision. In a 1988 prospective study, Rizzo and Lessell4 established that patients who appeared to have idiopathic optic neuritis have a very high risk of developing clinical MS. The ONTT was conceived to define precisely the clinical profile of patients with optic neuritis, to determine the value of corticosteroid treatment, and to determine the risk of developing MS.5 With regard to the impact of treatment on vision, it should be noted that unlike many other clinical trials, there was little at stake because it had long been recognized that the vast majority of patients with optic neuritis would enjoy visual improvement even without treatment. On the other hand, the trial's impeccably obtained data have standardized our approach to the diagnosis, treatment, and subsequent follow-up in patients with optic neuritis. Four of the trial's major findings—the prevalence of fellow eye abnormalities, types of visual field defects, adverse effects of oral corticosteroid treatment,6 and delay in onset of MS in patients treated with intravenous corticosteroids—were all unanticipated. Though still controversial, each of these findings has altered our approach to patients with this disorder, provoked much debate, and spawned important subsequent investigations.