Progressive multifocal leukoencephalopathy (PML) is a fatal demyelinating disease of the central nervous system that is caused by John Cunningham virus (JCV). It occurs almost exclusively in immunosuppressed individuals, for example, patients with AIDS and hematological and lymphoreticular malignancies. In this article, we present a review of the literature and 2 case reports with PML. The first report examines a 15-year-old male (who presented with dedicator of cytokinesis 8 deficiency) who was diagnosed as having PML based on characteristic magnetic resonance imaging (MRI) lesions and a positive PCR for JCV in cerebrospinal fluid. He was transferred for bone marrow transplantation after stabilization with therapy of maraviroc and cidofovir. The second report examines a 6-year-old male who presented with encephalitis and was also diagnosed with AIDS. He was diagnosed with PML and started treatment with cidofovir. His clinical status and MRI findings deteriorated rapidly. In immunosuppressive patients who developed encephalopathy, JCV ought to be considered.