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The TYSABRI® Observational Program (TOP) is an ongoing, global open-label study enrolling natalizumab-treated relapsing-remitting multiple sclerosis patients in real-world settings. Country-specific data can provide information on natalizumab’s benefit-risk in local clinical practice.Clinical outcomes in UK and rest of world (ROW) TOP patients were compared.As of May 2016, 134 UK and 5793 ROW patients were enrolled in TOP. Mean baseline Expanded Disability Status Scale (EDSS) score was 4.27 in the UK and 3.45 in ROW. Mean years on natalizumab was 3.52 in the UK and 3.04 in ROW. Annualized relapse rate (ARR) decreased by 89.7% (from 2.21 pre-natalizumab to 0.23 post natalizumab initiation; p<0.0001) in the UK and by 89.0% (from 1.99 to 0.22; p<0.0001) in ROW. In both cohorts, ARR decrease by baseline EDSS score was 91.0%–93.2% for <3.0 and 87.3%–88.9% for ≥3.0; mean EDSS score change from baseline over 6 years was <1.0. Overall, 9 of 134 UK patients (6.7%) experienced ≥1 serious adverse event.Natalizumab demonstrated similar effectiveness in the UK and ROW cohorts. Safety in the UK cohort was consistent with natalizumab’s established safety profile.BiogenRN: grant/travel support from Biogen; KR, SL, SFSF, CS: Biogen employees.