Progressive multi-focal leukoencephalopathy (PML) is a demyelinating disease of the central nervous system caused by reactivation of John Cunningham virus. It is an opportunistic infection affecting patients who are severely immunocompromised due to an underlying disease or secondary to immunosuppressive therapy. To date, no effective antiviral therapy has been established, though several substances are being investigated. The authors present the case of a 39-year-old previously healthy patient who was diagnosed with PML and sarcoidosis stage III without having received prior immunosuppressive treatment. The patient did not respond to treatment with mirtazapine, which has been used empirically, and deceased shortly after diagnosis.