An incidence cohort consisting of 308 multiple sclerosis patients was followed up repeatedly during at least 25 years of disease. A number of clinical factors were analysed with respect to their validity in assessing the long-term prognosis. Of the onset characteristics, the type of course was the most important, with primary progressive patients experiencing a much more severe course. In patients with an acute onset, low onset age, high degree of remission at first exacerbation, symptoms from afferent nerve fibres and onset symptoms from only one region (as compared with polyregional symptoms) of the central nervous system, were factors significantly associated with a favourable long-term prognosis. Of factors known 5 years after onset, a low number of affected neurological systems, a low neurological deficit score and a high degree of remission from the last bout were the most important favourable prognostic factors.