['To determine whether an increase in plasma concentration of S-100 protein can serve as a marker for acute exacerbation of multiple sclerosis.', 'The plasma level of S-100 protein was investigated in 28 patients suffering from multiple sclerosis. Of these, 17 patients were admitted for acute exacerbation and 11 patients had a stable disease with no clinical signs for acute exacerbation. S-100 protein concentrations in plasma were determined with an immunofluorometric sandwich assay.', 'Plasma concentrations were significantly elevated in patients who were examined within 7 days after the onset of acute exacerbation (n = 6). S-100 levels of patients 8 to 28 days after the onset of acute exacerbation (n = 11) did not differ from healthy controls (n = 120). Eleven patients with multiple sclerosis without acute exacerbation had moderately elevated plasma levels.'] Does acute exacerbation of multiple sclerosis increase plasma levels of S-100 protein?