Our main objective is to prove that extending dose intervals guided by natalizumab serum concentrations, will not result in radiological or clinical disease activity in completely stable RRMS patients treated with natalizumab. Adult patients with RRMS with no evident disease activity during the last 12 months of natalizumab infusions with a minimum treatment of 12 months wil be included. Before subsequent natalizumab infusions, serum concentrations will be evaluated. If the concentration exceeds 15μg/ml the dose interval will be extended with a week to a maximum of eight weeks. Patients will get regular brain MRI scans and clinical follow-up.
