Eleven patients with a relapsing-remitting form of multiple sclerosis (MS) were examined clinically and with magnetic resonance imaging scans 3 weeks before, at the day of vaccination with killed influenza virus and 3 weeks afterwards. No exacerbations were noted in the pre- or postvaccination period. Eight contrast-enhanced or active lesions were present at the onset of the study. Three new active lesions appeared at the end of the prevaccination period while only 1 new active lesion was found at the end of the postvaccination period. We conclude that vaccination with killed influenza virus has no clinical or subclinical short-term effect on the activity of MS.