In a pilot trial, eight patients with multiple sclerosis were matched to control patients and received T-cell vaccination with irradiated T cells reactive to myelin basic protein (MBP) to deplete circulating MBP-reactive T cells. In the 2 years before and after vaccination, exacerbations decreased in five vaccinated patients (numbers 1, 2, 6-8) with relapsing-remitting disease from sixteen to three, respectively, and from twelve to ten in their matched controls. Magnetic resonance imaging showed a mean 8.0% increase in brain lesion size in the vaccinated patients compared with a 39.5% increase in the controls. Lesions and/or relapses worsened in three cases after vaccination in association with reappearance of circulating MBP-reactive T cells.