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Background: Adherence to disease-modifying drugs (DMDs) is one of the key factors for achieving optimal clinical outcomes. Rebismart® is an injection device for subcutaneous administration of interferon beta-1a (INF β-1a) that is also able to monitor adherence objectively. The aim of this study was to describe adherence to INF β-1a using the said electronic autoinjection device and to explore the relationship between adherence and relapses in a Spanish cohort. Methods: This is a retrospective observational study in which 110 Spanish patients self-administered INF β-1a subcutaneously using an electronic autoinjection device between June 2010 and June 2015. The primary end point was the percentage of adherence measured by Rebismart® to subcutaneous INF β-1a injections calculated as number of injections received in time period versus number of injections scheduled in time period. Other variables recorded were demographic and clinical data. Statistical analysis was performed using SPSS 19.0 software. Results: Median adherence for the total study period was 96.5% (interquartile range [IQR]: 91.1-99.1). Similar values were observed during the first 6 months: 98.7% (IQR: 91.3-100), and the last 6 months: 97.6% (IQR: 91.1-99.8). Median duration of treatment was 979 days (IQR: 613.8-1,266.8). During the entire treatment period, 77.3% of patients were relapse free and mean annualized relapse rate was 0.14 (standard deviation: 0.33). Increased adherence was associated with better clinical outcomes, leading to lower relapse risk (odds ratio: 0.953; 95% confidence interval: 0.912-0.995). Specifically, every percentage unit increase in adherence resulted in a 4.7% decrease in relapse. Conclusion: Patients with multiple sclerosis who self-injected INF β-1a with Rebismart® had excellent adherence, correlating with a high proportion of relapse-free patients and very low annualized relapse rate. [ABSTRACT FROM AUTHOR]