OBJECTIVES: This study examined the relationship between medication adherence and outcomes in multiple sclerosis (MS), where both concepts are more difficult to measure than in diseases like hypertension where most medications are taken orally and surrogate outcomes (e.g., blood pressure) are routinely collected. METHODS: MS patients age ≥18 years treated at Geisinger Clinic and taking an MS medication were surveyed three times, six months apart, to assess medication adherence and MS outcomes. Patients reported their doses taken in the past month (adherence), number of relapses, TQSM medication satisfaction score, and MSIS physical and psychological functional scores.  Nonparametric bootstrap analyses were used to compare mean outcome scores among patients with and without missed doses.  A sub-analysis on patients with Geisinger insurance claims was conducted to calculate patient's MS medication possession ratio (MPR) from 2004 to 2013 and compare this claim-based adherence measure with the other results. RESULTS: