Progressive multifocal leukoencephalopathy (PML) is a demyelinating disease by John Cunningham virus (JC virus) infection in oligodendrocytes. The radiographic and clinical features, along with the identification JC in cerebrospinal fluid polymerase chain reaction, are sufficient for the diagnosis of PML in immunodeficiency. However, it is difficult to suspect PML without the patient history of immunodeficiency. A 32-year-old man presented with headache for a month without any medical history. Based on clinical and image features, the differential diagnoses included demyelinating lesion and neoplasms. Microscopically, biopsy specimen showed multifocal demyelinating and degenerative white matter, consistent with PML. Oligodendrocytes cells with increased nuclei and plum-colored inclusions were admixed with perivascular lymphocytic and histiocytic infiltration, and loss of myelin. Atypical astrocytes had large or multiple nuclei. After brain biopsy, human immunodeficiency virus infection was confirmed by serum chemiluminescent immunoassay. It is unlikely that PML would be considered without the information of immunosuppression. Therefore, it is very important to be aware of the histological features of PML.