In a hospital-based study of 400 patients with multiple sclerosis (MS), 42 per cent of patients who had had MS for 10 years or more had benign disease. Early age of onset and a long first remission were significantly associated with a good prognosis. There was a suggestion that initial presentation with paraesthesiae and possibly optic neuritis were associated with a benign prognosis, but the only significant finding was the association between limb weakness and a poor outcome (p less than 0.05). Fewer patients with benign disease had a progressive element to their disease than those in the more disabled group (p less than 0.001). The only laboratory test which was associated with a benign prognosis was the absence of CSF myelin basic protein in remission. Abnormalities of visual evoked response, CSF IgG and peripheral blood T lymphocytes appeared to have no value in assessing prognosis in the patients studied.