Progressive multifocal leukoencephalopathy is caused by JC virus and mainly affects human immunodeficiency virus infected patients. The disease is exceptionally rare in children. The diagnosis is based on magnetic resonance imaging and on the detection of JC virus DNA in the cerebrospinal fluid. This is a report of a 14 year old girl, not previously known to have human immunodeficiency virus, who was admitted to the hospital in a semicomatosed state after complaining of progressive neurologic symptoms for 4 months. Initially she complained of being tired and had an unsteady gait. This was followed by worsening myoclonic and generalized tonic-clonic convulsions. Brain magnetic resonance imaging revealed asymmetrical, diffuse subcortical white matter demyelination. Polymerase chain reaction test of the blood was positive for JC virus. Despite the initiation of antireteroviral therapy, the patient died in the intensive care unit 42 days after her admission.