Progressive multifocal leukoencephalopathy (PML) is a severe encephalic demyelinating disease associated with JC virus (JCV) reactivation that occurs mostly in patients with immune disorders. Patients affected by sarcoidosis are at risk for developing PML both for leukocyte dysfunction and for receiving immunosuppressive medications: delayed diagnosis and high-dose corticosteroids are associated with a reduced survival. Although no specific treatment for PML exists, several therapeutic possibilities have been assessed with uncertain benefits (5HT2a receptor inhibitors are active in vitro against JCV): the cornerstone of sarcoidosis-associated PML is immunosuppressants withdrawal.We report the case of a female patient affected by systemic sarcoidosis for 30 years receiving low-dose corticosteroids (5 mg every other day). Due to memory impairment an MRI was performed showing three T2 hyperintense lesions involving white matter. Cerebrospinal fluid JCV PCR (845 copies/ml), neuropathological examination and immunohistochemistry (SV40 protein and JCV DNA positivity) on brain biopsy confirmed PML. Mirtazapine 15 mg was started while prednisone treatment was continued. 3 and 6 months later cognitive performances improved and brain MRIs were stable while cerebrospinal fluid JCV DNA was undetectable (6 months later). In conclusion the diagnosis of PML in patients with sarcoidosis is challenging given the overlapping presentation; the use of 5HT2a receptor antagonists deserves further studying in patients needing immunosuppressant drugs to control their dysimmune disease.