Background: Interferon beta-1b (IFNB-1b; Betaferon registered )has demonstrated efficacy and safety in relapsing-remitting MS (RRMS). It was the first approved interferon in France for RRMS with greater than or equal to 2 relapses in the last 2 years. Impact of IFNB-1b on very active MS (VAMS) has not been studied. The 12-year long-term follow-up (LTF) French cohort ATU of IFNB-1b will evaluate the long-term effect of IFNB-1b on clinical parameters in a cohort of patients with VAMS. Objective: To assess the clinical status MS patients included in the French cohort ATU of IFNB-1b after 12-year follow-up. Methods: This study is a multi-center, open-label, observational study that evaluates outcomes in patients included in the French cohort of ATU between July 1995 and June 1996. ATU is an administrative procedure aimed to propose treatment to patients before definite agreement by the French health regulatory agency. During this period, inclusion criteria for IFNB-1b were: 1) RR clinically definite MS; 2) greater than or equal to 3 relapses with objective sequellae confirmed by a neurologist in the last 12 months; 3) Expanded Disability Status Scale (EDSS) score: 3.0 less than or equal to EDSS less than or equal to 6.0; 4) Age: 18-50 years; 5) MS onset <10 years; 6) Abnormal magnetic resonce imaging (MRI). When a neurologist wanted to treat a patient he sent the medical record to a selection comittee of 12 members, who checked the inclusion criteria and allowed the delivery of IFNB-1b. All the patients were followed by a neurologist for safety end efficacy every month for the first 3 months and then every 3 months. The procedure of the ATU ended when IFNB-1b was marketed in August 1996. All the patients included in the cohort ATU were identified. Their medical history was reviewed to collect the following information: duration of treatment, reason for treatment interruption, survival, course of the disease (relapsing-remitting or secondary progressive), relapse rate, EDSS score. Results: Two hundred and thirty seven MS patients were included in the cohort. At inclusion, mean age was 34 years, mean MS duration was 5 years, mean relapse rate was 3.5; median EDSS was 4.0. Follow-up data will be presented at the meeting. Conclusions: The French cohort ATU is the first LTF of VAMS patients under IFNB-1b.