We found 42 of 74 patients (57%) with isolated monosymptomatic optic neuritis to have 1 to 20 brain lesions, by magnetic resonance imaging (MRI). All of the brain lesions were clinically silent and had characteristics consistent with multiple sclerosis (MS). None of the patients had ever experienced neurologic symptoms prior to the episode of optic neuritis. During 5.6 years of follow-up, 21 patients (28%) developed definite MS on clinical grounds. Sixteen of the 21 converting patients (76%) had abnormal MRIs; the other 5 (24%) had MRIs that were normal initially (when they had optic neuritis only) and when repeated after they had developed clinical MS in 4 of the 5. Of the 53 patients who have not developed clinically definite MS, 26 (49%) have abnormal MRIs and 27 (51%) have normal MRIs. The finding of an abnormal MRI at the time of optic neuritis was significantly related to the subsequent development of MS on clinical grounds, but interpretation of the strength of that relationship must be tempered by the fact that some of the converting patients had normal MRIs and approximately half of the patients who did not develop clinical MS had abnormal MRIs. We found that abnormal IgG levels in the cerebrospinal fluid correlated more strongly than abnormal MRIs with the subsequent development of clinically definite MS.