Background: Multiple sclerosis (MS) is a neurologic disorder with a considerable global burden. During the last decades, some pharmaceutical treatments have been approved for patients with MS. Dimethyl fumarate (DMF) is one of these drugs which has been reported to have early promising results in recent studies, but the efficacy of this drug in patients with MS is still being studied in different parts of the world. In the present study, we evaluated the effectiveness of DMF therapy on reducing relapses, lesions, and disability in Iranian patients with MS.Methods: The present single-arm before-after study was approved by the Ethics Committee of Mashhad University of Medical Sciences, Mashhad, Iran [Iranian Registry of Clinical Trial (IRCT) code: IRCT20190121042439N1]. Every patient who was diagnosed with relapsing MS was considered eligible to enroll in the present clinical trial. Before receiving DMF therapy, the baseline liver function tests and complete blood count were obtained from all individuals. Also, a baseline brain magnetic resonance imaging (MRI) was obtained and Expanded Disability Status Scale (EDSS) was documented from all patients. After receiving 240 mg DMF twice daily for 12 months, the laboratory and imaging measurements as well as EDSS were repeated. Furthermore, the total number of relapses within the study period was recorded. Satisfaction with DMF treatment was determined by answering a yes-no question.Results: A total number of 50 patients enrolled in the study and most of them were female (80%). There was a significant decrease in EDSS score and gadolinium (GD)-enhancing lesions after the study period (P < 0.001 for each). Moreover, the attacks significantly dropped after the study period (P < 0.001) and 86% of patients were satisfied with their treatment.Conclusion: The findings of this study showed that 240 mg DMF administered twice daily can effectively reduce disability and provide satisfaction within the first year of therapy in patients with MS.