The diagnosis of optic neuritis is based on clinical signs and symptoms. Ancillary testing is of little medical value, but helpful to evaluate the risk of developing multiple sclerosis and for proper counseling of the patient. The visual field defects caused by optic neuritis are less often central, but most often altitudinal or sectorial, and thus of little help to differentiate between papillitis and anterior ischemic optic neuropathy. In contrast to common belief, the usual therapy with oral prednisone at an initial dose of 100 mg/day may be harmful. This therapy does not accelerate recovery of visual function and does not lead to a better final result but increases the risk of new episodes of neuritis and may favour the development of multiple sclerosis. In the case of severe visual loss (visual acuity < or = 0.1) a megadose-therapy with 1000 mg methylprednisolone/day accelerates the recovery of visual function. Side effects of the megadose therapy are infrequent and not severe. In the case of moderate visual loss (visual acuity > or = 0.5) no therapy is advocated.