Background: Natalizumab is a drug used in multiple sclerosis (MS) and its main side effect is the development of progressive multifocal leukoencephalopathy (PML). Since this is potentially fatal or disabling, treatment must be stopped immediately if it is suspected, taking into account the possible later development of immune reconstitution syndrome or renewed exacerbation of MS.

Methods: We report a case of initially asymptomatic PML within the context of treatment with natalizumab in a female patient with MS. High antibody titers to the John Cunningham virus (JCV) and over two years' treatment were established as risk factors. The polymerase chain reaction for the JCV in cerebrospinal fluid was negative in two determinations. The interval between the radiological diagnosis and the onset of the clinical features was two months. During the course of the disease, the patient developed immune reconstitution inflammatory syndrome and relapses, or renewed exacerbation, of her MS. She responded well after beginning treatment with fingolimod, once the PML had become stabilised.

Conclusions: This case indicates the importance of close clinico-radiological monitoring in patients with MS treated with natalizumab, especially when they present risk factors for the development of PML, as well as its potential incidence on survival and final functional status.