OBJECTIVES: Global wellbeing of patients is an important outcome in research and clinical practice. Multiple Sclerosis (MS) is a degenerative, neurological condition, characterised by progressive disability, affecting approximately 110 per 100,000 people in England and Wales. Natalizumab is a humanised monoclonal antibody, licensed for use in highly active relapsing-remitting MS and administered as an IV infusion every 28 days. The purpose of this study was to explore the impact of natalizumab on both patients and use of health service resource in clinical practice in England. METHODS: Health Episode Statistics (HES) data were used to perform a retrospective cohort study. A structured coding search elucidated a comprehensive list of natalizumab users by hospital trust. Analysis of health service usage, including outpatient appointments and admissions, was undertaken. Comparison of use during the year before and after treatment initiation was conducted.  RESULTS: 2,196 patients with at least 1 year of available HES data after treatment initiation were identified. In this cohort, natalizumab treatment was associated with 38% fewer unplanned admissions (981 vs. 604, Χ12 = 89.2, p<0.001) and 58% fewer unplanned bed nights (8,817 vs. 3,681, Χ12 = 2109.8, p<0.001) in the year following treatment initiation compared with the year before. A 14% reduction in all outpatient appointments was also observed, due principally to a 25% reduction in neurological outpatient appointments (from 7,826 to 5,901, Χ12 = 269.7, p<0.001). Evidence from this study indicates that duration of treatment is a significant factor in this response; patients receiving between 12 and 14 doses (n=1,122) experienced 70% reduction in unplanned admissions and 90% reduction in unplanned bed nights.  CONCLUSIONS: Our data support the notion that natalizumab treatment significantly reduces unplanned hospital treatment and outpatient attendance. The abstract authors are employees of Biogen Idec, analysis has been funded by Biogen Idec, medical writing support provided by Insight2Implement Ltd