Progressive multifocal leukoencephalopathy (PML) is a very rare and often fatal demyelinating disease of central nervous system (CNS), which mostly occurs in patients with immunosuppression such as acquired immunodeficiency syndrome (AIDS) patients, transplant patients and patients receiving chemotherapy. PML usually manifests with acute or subacute neurologic deficit. and its late diagnosis may lead death or significant permanent disability. We report a 33-year old man diagnosed with PML in AIDS, who initially presented with gradual onset of dementia. Most symptoms of PML were progressed rapidly for several months, and characterized by focal neurological symptoms. On the other hand, we were experienced in patients without focal neurological symptoms and ongoing overall cognitive decline slowly. Patients with immunosuppression can be presented in a variety of neurological symptoms, detailed examinations for cognitive functions were needed in early stage of the disease.