We report a case of a 53-year-old woman with refractory rheumatoid arthritis (RA), which is resistant to standard therapy, treated with synthetic disease modifying antirheumatic drugs (DMARDs) in monotherapy as well as combined therapy (e.g. leflunomide, methotrexate, cyclosporine, sulfasalazin) and steroids, who developed symptoms of rapidly progressive dementia after infliximab administration. Due to the recent increase in the use of biological drugs, not only in the area of rheumatology, more and more reports about their side effects have been submitted, which involve infections and central nervous system disturbances. Neurological disturbances were observed in the course of therapy with all types of TNF- inhibitors. They include: paraesthesiae (65%), visual disturbances associated with ocular nerve inflammation (40%) and confusion (25%). Moreover, available literature data suggest that patients with RA treated with immunosuppressive therapy, incur a higher risk of either serious infections or activation of latent virus infections like Herpes or JC virus than the healthy population. In view of a potential connection of severe cognitive disorder with infliximab therapy, we made an assumption of progressive multifocal leucoencephalopathy (PML) resulting from activated latent JC virus infection. Magnetic resonance (FLAIR) image has shown hyperintensive lesions localized in white and gray matter of temporal and frontal lobes, in both cerebral hemispheres (Fig. 1). This localization was typical of herpes simplex encephalitis (HSE), but it is currently known that very similar lesions can be seen in PML. Both neuroinfections, HSE and PML can manifest as dementia. Because of a negative spino-cerebral fluid Herpes Simplex Virus PCR (polymerase chain reaction) testing result, we decided to perform a brain biopsy. HSE was diagnosed on the basis of a histopathological examination. After discontinuation of infliximab and introduction of antiviral therapy with aciclovir the patient’s neurological status improved.