OBJECTIVES: To determine if patients are adherent with interferons used to treat patients with Multiple Sclerosis (MS), the optimal adherence rate to maximize clinical outcomes, and the impact of adherence on the number and severity of relapses.    METHODS: A retrospective cohort study design was used. Pharmacy and medical claims data were extracted for 2006-2008. Adherence was measured using two standard methods for Medication Possession Ratio (MPR), one that incorporates persistence and one that does not. Threshold analyses were run to determine an optimal adherence rate to minimize relapses. Patients were considered adherent if they met a predefined standard cut-off point and reanalyzed with the newly determined cut point. The number and severity of relapses were measured for each patient year, with 2008 being the critical outcomes point. A series of regression models were used to assess the impact of adherence on the number and severity of relapses.   RESULTS: A total of 3590 patients were included in the study. Based on the method, the average MPR varied between 77.6% and 89.8%. The threshold analysis determined that the optimal adherence rate is around 85%. Patients receiving interferon through a specialty pharmacy are more adherent than those who did not. Patients who were classified as adherent through 2007 had a significantly lower risk of relapse in 2008. Patients who were adherent also had a significantly lower risk of severe relapse than those who were non-adherent. Complete MPRs, adherence rates, individual year descriptives, and inferential statistics will be included in the presentation.  CONCLUSIONS: Patients with MS are generally adherent with their interferon therapy, although opportunities for improvement exist. Patients who receive special pharmacy services are more adherent than those who receive standard retail services. The effect of adherence on the number and severity of relapses demonstrated the positive impact of interferons for MS treatment.