Marieke Krol,1 Gert de Voer,2 Ulrike Osowski3 1QuintilesIMS, Capelle aan den Ijssel, the Netherlands; 2Merck B.V., Schiphol-Rijk, the Netherlands; 3Merck KGaA, Darmstadt, Germany Purpose: Long-term treatment adherence among patients with multiple sclerosis (MS) is a general concern, with an established correlation with clinical efficacy. Closely monitoring patients’ treatment behavior may have a beneficial effect on adherence. This study assessed adherence, in daily life, to subcutaneous (sc) IFN beta-1a, self-administered using the RebiSmart® electronic injection device (the IFN beta-Ia autoinjector device), in patients with MS. Patients and methods: This was a retrospective observational study analyzing treatment adherence based on injection data, eg, injection date and dose, extracted from the IFN beta-Ia autoinjector devices collected from patients in Germany and the Netherlands. Results: Data recorded in the period from 2007 to 2012 by the IFN beta-Ia autoinjector devices from 1,682 (79.7% from Germany, 20.3% from the Netherlands) patients were analyzed. A mean of 94.8% of the multi-dose cartridges (containing sc IFN beta-1a for three injections) were used completely, indicating a low incidence of application errors and drug wastage. The mean adherence rate was 90.7% and 82.9% over the entire observation period (mean treatment duration: 150.1 weeks). Median adherence rates were similar between German and Dutch patients (97.9% vs 99.0%). Conclusion: In daily clinical practice, patients using the IFN beta-Ia autoinjector device were highly adherent to sc IFN beta-1a. The injection data stored electronically in the device may help patients to adhere to treatment regimens and, if viewed by physicians, promote discussion of adherence issues with patients. Keywords: multiple sclerosis, injection device, IFN beta-1a, observational study