The cerebral hemispheres of 19 patients with acute monosymptomatic optic neuritis (AMON) were investigated using magnetic resonance imaging (MRI) and Gadolinium-DTPA (Gd-DTPA). Using T1-weighted and T2-weighted imaging sequences it was disclosed that 14 of 19 patients had lesions and that enhancement was seen in seven patients. It is known that patients with AMON and silent lesions in the CNS have a highly increased risk of developing multiple sclerosis (MS) later on. If it is accepted that a silent lesion in patients with AMON does represent a multiple sclerosis (MS) plaque, and that an enhancing silent lesion represents an acute MS lesion, we may hypothesize that the disease process ultimately leading to MS starts long before the first symptom or sign ever appears. It would seem that at least half of the silent lesions in the cerebral hemispheres of patients with AMON had existed before the onset of the AMON, and that the disease actually started before the onset of the AMON.