In a prospective study of hitherto 70 patients with acute optic neuritis (ON), 18 patients aged 15-49 years (12 women, 6 men) were diagnosed as having very subtle form of ON (bilateral in 4 patients), characterized by normal visual acuity. However, their symptoms were sudden functional visual disturbances, most frequently blurred vision accompanied by pain in or around the affected eye(s). In 5 patients, the ON was a manifestation of clinically definite multiple sclerosis (MS); the remaining 13 patients had monosymptomatic ON. The duration of visual symptoms ranged from 2 to 28 days (median 7 days) at the time of examination. Although 18 patients had a normal visual acuity, i.e. 6/6 c.c. (Snellen's notation) or better, extensive studies of the visual functions (using sensitive supplementary tests) revealed various abnormalities, primarily various visual field defects, abnormal contrast sensitivity, abnormal VEP and colour vision deficiencies (often of blue-yellow type). Magnetic resonance imaging (MRI) of the brain revealed demyelinating lesions in 10 of the 13 patients with monosymptomatic ON, and in all 5 patients with definite MS. The extended disease spectrum gives reason to hypothesixe that ON may occur more frequently than previously reported, and that the described subtle form of ON could be an unnotified precocious manifestation of the demyelinating disease.