Wijburg MT, et al. - An inquiry was set up regarding the correlation of progressive multifocal leukoencephalopathy (PML) lesion characteristics on magnetic resonance imaging (MRI) with both qualitative and quantitative JC virus (JCV) polymerase chain reaction (PCR) results in cerebrospinal fluid (CSF) of patients with natalizumab-associated PML (NTZ-PML). Results shed light on the connection between smaller NTZ-PML lesions with a higher tendency of undetectable JCV DNA in CSF. This could preclude a formal diagnosis of PML. Furthermore, it could complicate the patient treatment in subjects with small MRI lesions highly indicative of PML detected early via pharmacovigilance.