In patients with either relapsing-remitting or secondary-progressive multiple sclerosis, there were fewer new lesions/patient with natalizumab (0.7 and 1.1 with natalizumab 3 and 6 mg/kg every 28 days, respectively) than in the placebo group (9.6 new lesions/patient) over 6 months. There were also fewer relapses in the natalizumab groups than the placebo group. However, there were no changes in the Expanded Disability Status Scale scores in any of the groups. Natalizumab was well-tolerated. Thus, the initial results with natalizumab treatment over 6 months in multiple sclerosis are encouraging.