Background Lack of adherence to medication in rheumatoid arthritis (RA) can worsen disease activity, but data on adherence to biological drugs is scarce. Objectives To evaluate RA patients adherence to the subcutaneous (SC) biological treatment prescribed by the physician during the first year of treatment. Methods Retrospective study in RA patients ≥18 years-old from 42 Spanish hospitals, which had been prescribed a SC biological drug 12 to 18 months prior to the study visit. Data were collected from patients' clinical records and hospital pharmacy drug-dispensing logs. Adherence was evaluated through the Medication Possession Ratio (MPR) (actual number of days covered by the taken medication/theoretical number of days covered by the prescribed medication). The number of days theoretically covered was calculated considering the interval of administration of the prescribed biological drug, if induction dose was prescribed, and if there were suspension periods or changes in the dose. The actual number of days covered was calculated with the number of vials that the patient had taken from the hospital pharmacy during the study period. Non-adherence was diagnosed if MPR was ≤80%. Results 364 patients were included (age 54.9 years [12.5]; 77.5% women, median RA duration 7.8 years). The initial interval of administration of the biological drug was weekly (44.2%), biweekly (39.1%) and monthly (17.3%); the median duration of the period studied was 14.8 months. For the primary analysis, 363 patients were valid, of which 52 patients were non-adherent (MPR ≤80%) to the prescribed schedule (14.3%; 95% CI: 11.1–18.3). There were no differences in adherence by age ranges, gender or other socio-demographic factors, such as level of education, smoking habit or alcohol intake, employment status, comorbidities or polypharmacy (all, p= NS). Non-adherence was less frequent in patients with a monthly administration schedule (6.4%) than in patients with a weekly (17.4%, p=0.034) or biweekly administration (14.4%, p=0.102), and slightly more frequent in those receiving induction (21.6% vs 12.5% without induction, p=0.068). No differences were observed between patients who used syringes or pen devices, or in patients who had periods of treatment interruption. Conclusions In RA patients, 14.3% were non-adherent to the prescribed SC biological medication during the first year of treatment, with no differences related to socio-demographic variables. Adherence was higher in those patients with monthly subcutaneous administration. Acknowledgement The ARCO study was funded by Merck Sharp & Dohme of Spain. Disclosure of Interest None declared