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Background: A significant proportion of pediatric-onset multiple sclerosis (POMS) patients do not respond to firstline disease-modifying therapies. Clinical trials showed that natalizumab is effective and safe in adults, but there are limited clinical trial data for children. Natalizumab is currently prescribed off-label for POMS. We aimed to characterize the effectiveness, safety and tolerability of natalizumab in all POMS cases treated in Portugal (from 2007 to 2018). Methods: Data from clinical records were retrospectively collected for all POMS cases treated with natalizumab in Portugal.Results: Twenty-one patients were included, 14 (67%) of which were female. The median age at POMS diagnosis was 13 years old. The median duration of treatment with natalizumab was 2 years and 3 months. Median Expanded Disability Status Scale score decreased from 1.5 to 1.0 after 24 months. The Annualized Relapse Rate decreased from 1.31 events/patient/year before treatment with natalizumab to 0 after 12 months of treatment and to 0.04 after 24 months. No gadolinium-enhancing lesions or new or enlarged T2 hyperintense lesions were observed in 8/8 patients (100%) after 12 months, and 4/5 (80%) after 24 months. There was one possible serious adverse event, which did not require dose adjustment. Five patients discontinued treatment due to positive anti-JCV (JC virus) antibody JC serostatus. Conclusion: Natalizumab may be an effective and safe disease-modifying therapy for POMS. Our results are in line with data published for the adult population, as well as with similar observational studies in pediatric populations in other regions.