Background: African Americans (AAs) with multiple sclerosis (MS) seem to have a more severe disease course than white Americans (WAs). To our knowledge, it is not known to what extent treatment with interferon beta-1a will effect the MS disease course within the AA population. Objective: To compare the response to treatment with interferon beta-1a between AA and WA MS patients. Design: This is an exploratory post hoc analysis of the Evidence of Interferon Dose-Response: European North American Comparative Efficacy (EVIDENCE) study. Setting: The EVID ENCE study is a randomized controlled trial that compared the efficacy of once weekly, intramuscular, 30-μg interferon beta-1a treatment with thrice weekly, subcutaneous, 44-μg interferon beta-1a therapy in treatment-nave MS subjects. Participants: Thirty-six AA subjects were compared with 616 WA subjects. Main Outcome Measures: The number of MS exacerbatio ns, the proportion of exacerbation-free subjects, and the number of new MS lesions present on brain magnetic resonance imaging were compared between AA and WA subjects at 24 and 48 weeks after initiating treatment with interferon beta-1a. Results: The AA subjects experienced more exacerbations and were less likely to remain exacerbation free (statistical trends). The AA subjects developed more new MS lesions on T2-weighted brain magnetic resonance imaging at 48 weeks (P=.04). Conclusions: Despite the small sample size, AA subjects appeared less responsive to treatment than WA subj ects on outcome measures, reaching significance only for T2-weighted lesion count at 48 weeks. However, it is difficult to base these differences solely on response to treatment given the potential differing in MS disease course in AA patients. Background: African Americans (AAs) with multiple sclerosis (MS) seem to have a more severe disease course than white Americans (WAs). To our knowledge, it is not known to what extent treatment with interferon beta-1a will effect the MS disease course within the AA population. Objective: To compare the response to treatment with interferon beta-1a between AA and WA MS patients. Design: This is an exploratory post hoc analysis of the Evidence of Interferon Dose-Response: European North American Comparative Efficacy ( EVIDENCE) study. Setting: The EVID ENCE study is a randomized controlled trial that compared the efficacy of once weekly intramuscular 30-μg interferon beta-1a treatment with thrice weekly, subcutaneous, 44-μg interferon beta-1a therapy in treatment- Participants: Thirty-six AA subjects were compared with 616 WA subjects. Main Outcome Measures: The number of MS exacerbations ns, the proportion of exacerbation-free subjects, and the number of new MS lesi ons present on brain magnetic resonance imaging were compared between AA and WA subjects at 24 and 48 weeks after initiating treatment with interferon beta-1a. Results: The AA subjects experienced more exacerbations and were less likely to remain exacerbation free (statistical trends). The AA subjects developed more new MS lesions on T2-weighted brain magnetic resonance imaging at 48 weeks (P = .04). Conclusions: Despite the small sample size, AA subjects have less responsive to treatment than WA subj ects on outcome measures, reaching significance only for T2-weighted lesion count at 48 weeks. However, it is difficult to base these differences solely on response to treatment given the potential differing in MS disease course in AA patients.