Acute demyelinating optic neuritis (ON) is the initial presentation in approximately 20% of cases of multiple sclerosis (MS) and is characterized by unilateral, subacute, painful visual loss without systemic or neurological symptoms. The Optic Neuritis Treatment Trial (ONTT) has provided valuable insights into both the natural history and clinical course of demyelinating ON with respect to treatment. Visual function improves spontaneously over weeks and within 12 months 93% have recovered to a visual acuity of at least 20/40. Treatment with high-dose corticosteroids may accelerate visual recovery, but has little impact on long-term visual outcome. In the ONTT the 10-year risk of recurrence of demyelinating ON was 35%. The presence of white matter lesions on the initial magnetic resonance image of the brain has been identified as the strongest predictor for the development of MS. The 15-year risk of developing MS in the ONTT was 25% with no lesions, but 75% with one or more lesions. Since there is evidence of early axonal damage in acute demyelinating ON, disease-modifying drugs should be considered in patients at high risk of developing MS in the future as prophylaxis against permanent neurological impairment.