Kapica-Topczewska K, Collin F, Tarasiuk J, et al. - Given that presence of anti-JC-virus antibodies (JCVAbs) is correlated with the increased risk of natalizumab (NAT)-related progressive multifocal leukoencephalopathy (PML), researchers sought to evaluate the real risk of PML, seroconversion rate, and time to seroconversion in all JCVAb-negative relapsing-remitting multiple sclerosis (RRMS) patients treated with NAT in Poland. Using the Therapeutic Programme Monitoring System provided by National Health Fund (NFZ), demographic and clinical data of all Polish RRMS patients treated with NAT reimbursed by the NFZ were prospectively gathered in electronic files. There were 210 negative JCVAb RRMS patients with at least 9 (± 3) months of observation (146 females, 64 males, and the median age at baseline: 33 years) from Polish multiple sclerosis centers involved in the analysis. In JCV-seronegative RRMS patients, NAT therapy is safe and results in a lack of PML cases. JCV seroconversion rate in Poland is comparable to that noted in other European countries.