IFN-beta is effective in reducing relapses and magnetic resonance imaging (MRI) lesions in multiple sclerosis (MS). It is assumed that individual therapeutic responses vary, but methods to identify IFN-beta responsiveness have not been validated. Our objective was to evaluate methods to classify IFN-beta responder status using relapses and MRI lesions. Data was analyzed from 172 patients who were followed up in a placebo-controlled clinical trial of IFN-beta1a for 2 years. Patients were classified as responders or nonresponders using (1) the number of relapses during the 2-year trial; (2) the number of new T2 lesions after 2 years; and (3) the number of gadolinium-enhancing lesions at year 1 and year 2 on study. Outcomes included 2-year change in the Expanded Disability Status Scale, Multiple Sclerosis Functional Composite, and brain parenchymal fraction. We found that subgroups with high on-study relapse numbers had more disease progression, differences between responder subgroups were similar in the IFN-beta1a and placebo arms. In contrast, subgroups with high numbers of new MRI lesions had significantly more disease progression only in the IFN-beta1a arm. Baseline characteristics failed to account for differential outcome. New MRI lesion activity during IFN-beta1a treatment correlates with poor response to IFN-beta1a. MRI classification may facilitate rational therapeutic decisions, better clinical trial designs, and studies correlating biomarkers with therapeutic response.