15 patients with unilateral optic neuritis and 2 patients with bilateral optic neuritis were treated with 1000 mg methylprednisolone iv. per day for 5 days. In the cases of unilateral optic neuritis, visual acuity was reduced to -0.1, in those with bilateral optic neuritis to less-than-or-equal-to 0.6 in the better eye. The treatment was started one to 70 days after the onset of the neuritis. We examined whether vision recovered rapidly during the treatment. As a rapid recovery we defined a fourfold improvement on a logarithmic scale during the 5 days of methylprednisolone medication. Such a rapid recovery was found in 11 of the 15 patients with unilateral and in 1 of the 2 patients with bilateral optic neuritis. A similar recovery was not found before and after the treatment interval. Although we did not have a control group, the correlation in time between the therapy and the rapid recovery suggests that the megadose steroids were effective in our patients. This interpretation is compatible with the results of the randomized controlled multicenter trial of Beck et al. (New Engl. J. Med. 326:81, 1992): However, the beneficial effect was seen up to 6 months only; one year after treatment, visual functions did no longer differ between the megadose and the placebo groups. Low-dose oral steroids did not improve visual function at any time and carried a higher risk for new episodes of neuritis, compared to placebo. Therefore, the ''traditional'' low-dose steroid therapy for optic neuritis has become obsolete.