Clinical presentations, diagnosis and differential diagnosis of optic neuritis of different etiology are reviewed. This symptom is concomitant with the development of multiple sclerosis in about 30% cases and accompanies the disease in 75% patients. Acute or subacute lesion of the optic nerve can be a symptom of optic myelitis, Leber's disease, systemic lupus erythematosus, Susac's syndrome, idiopathic retinal vasculitis, sarcoidosis and some other diseases. Clinical features and course of optic neuritis in these diseases are discussed.