Ophthalmologists and neurologists from a catchment area of 1.5 million people were asked to refer all patients with symptoms and signs of optic neuritis (ON) to our neurological department immediately. A total of 68 consecutive patients were extensively screened for known etiologies of ON. Eleven had definite multiple sclerosis (MS); 7 patients could not participate for various reasons. The cerebrum, cerebellum and brainstem of the remaining 50 untreated patients with monosymptomatic ON were MRI scanned at 1.5 T with 2 sequences (slice thickness 4 mm, voxel size 1.2 x 1.2 x 4 mm3): double spin-echo (TR = 1.8 s, TE = 30 and 90 ms, 12 slices axially) and inversion recovery (TR = 2.45 s, TI = 400 ms, TE = 30 ms, 5 slices sagittaly). Magnetic resonance imaging (MRI) was performed within 3-49 (median 16) days from onset of symptoms. MRI demonstrated multiple asymptomatic central nervous system (CNS) lesions in 62% of patients. The appearance, locations and extents of lesions were consistent with demyelination. A scoring system based on number and size of lesions is proposed. Seven of 50 patients developed definite MS during a median follow-up of 11 months; they all had CNS lesions on the first scan. MRI has thus improved the pathophysiological knowledge of idiopathic ON, which from the very onset is a manifestation of MS in at least two thirds of cases. Patients who have early CNS lesions run a much higher risk of later developing clinically definite MS.