This is a multi-centre, double blind, placebo controlled, randomised, parallel group, phase 4 study. Following three months treatment with Interferon beta 1a (Avonex) patients will be randomised for treatment with simvastatin or placebo as an add-on to interferon -beta-1a (AvonexÒ). Patients will start treatment with 40 mg peroral simvastatin daily or identically appearing placebo for one month. Hereafter, patients will escalate dosage to 80 mg daily. The patients will be examined clinically at baseline and at, 3, 4, 6, 9, 12 and 15 months. Patients who attend visit 5 (15 months) before the last patient has attended this visit will be asked to attend additional visits (visits 6+) until the last patient has attended visit 5. Clinical examination will be performed, for applicable patients, at 3 month intervals until the end of the study. This will be a maximum of two years, i.e. no more than eight additional visits. Laboratory assessments will be performed at screening 3, 4, 6, 9, 12 and 15 months after baseline, and for applicable patients additionally at 3 month intervals until end of study. MRI will be performed (T1-weighted and T2-weighted) at randomisation (3 months after baseline) and 12 months hereafter.