Objective To estimate the risk of progressive multifocal leukoencephalopathy (PML) and the safety of natalizumab administration in patients with relapsing-remitting multiple sclerosis (RRMS).

Methods A descriptive retrospective observational study including all patients with RRMS treated with natalizumab followed-up after 10 years.The likelihood of developing PML was estimated based on three risk factors: anti-John Cunningham virus antibody index, previous immunosuppressive therapy, and duration of treatment. Patients were classified into five categories: minimum probability (<0.1/1000); low (0.1/1000); medium-low (0.2–0.6/1000); medium-high (0.8–3/1000); high probability (3–10/1000).

Results 34 patients were included. The probability of PML in the last cycle was: 55.9% minimum, 8.8% low, 11.8% medium-low, 3% medium-high, and 20.5% high. 12 patients continue with active treatment with natalizumab. No cases of PML have been confirmed. Adverse effects were detected in 50% of patients.

Conclusions Quantifying risk factors allows us to estimate the probability of PML appearance, thus assessing the maintenance or suspension of natalizumab.