OBJECTIVES: Adherence to disease-modifying drugs is one of the key factors for achieving optimal clinical outcomes. Rebismart^® is an injection device of subcutaneous interferon beta-1a (INF β-1a) which is able to monitor adherence objectively. The aim of this study was to describe adherence to INF β-1a using that electronic autoinjection device and to explore the relationship between adherence and relapses in a Spanish cohort. METHODS: Observational, retrospective study in which 110 Spanish patients self-administered INF β-1a subcutaneously via electronic autoinjection device between June 2010 and June 2015. The primary endpoint of the study was percentage of adherence measured by Rebismart^® to subcutaneous INF β-1a injections calculated as: number of injections received in time period vs number of injections scheduled in time period. Other variables recorded were demographic and clinical data. For statistical analysis the SPSS 19.0 software was used. RESULTS: Median adherence over all study period was 96.5% (interquartile range [IQR]: 91.1 to 99.1) and a higher value was observed across the first six months: 98.7% (IQR: 91.3-100). Median duration of treatment was 979 days (IQR: 613.8 to 1266.8). During all period treatment, 77.3% of patients were relapse-free and annualized relapse rate mean was 0,14 (standard deviation [SD]: 0.33). Increased adherence was associated with better clinical outcomes, leading to lower relapse risk (odds ratio [OR]: 0.953; 95% confidence interval [CI]: 0.912-0.995). Specifically, every percentage unit increase of adherence resulted on a 4.7% decrease of relapse. During the five years evaluated, 38.2 % (n=42) of patients discontinued treatment due to disease progression or relapse (11.8 %), adverse events (10.9%), pregnancy (8.2%) or change to oral treatments (7.3%). CONCLUSIONS: Patients with multiple sclerosis self-injecting INF β-1a with Rebismart^® had excellent adherence which was correlated with a high proportion of relapse-free patients and a very low annualized relapse rate.